Above: the life-cycle of the rove beetle Aleochara bilineata (based in part on Wadsworth's 1915 classic account of this insect).
Insects exhibit two different principal types of life-cycle. Most insects lay eggs (though a few give birth to live young) which are
deposited in soil or attached to vegetation or some appropriate food source. In some insects, the egg hatches into a nymph
that resembles the adult, differing perhaps slightly in form, but much smaller; these are the hemimetabolous insects.
Holometabolous insects are those whose eggs hatch into a larva that differs greatly from the adult (imago) in form.
The tough exoskeleton (cuticle) of insects restricts their growth. To overcome this restriction, insects shed their skin every so
often, or moult. The number of moults is precise and adults do not moult. When an insect moults, its new skin is soft and white
at first, this is an insect's most vulnerable period. The insect can then expand itself like an inflatable toy by taking in air or
water. The new cuticle subsequently darkens and hardens, and the newly formed spaces, that resulted from inflation, are filled
with blood and are slowly replaced by tissues, until the next moult. We talk of instars, where the first form, which hatches from
the egg, is the first instar larva or nymph. This moults into the second instar larva or nymph, and so on, until the adult stage is
reached.
In hemimetabolous insects, the nymph undergoes a gradual metamorphosis into the adult form, appearing more like the adult
with each moult, but these changes are slight, since the first instar nymph resembled the adult anyway, and the most obvious
change is an increase in size. The silverfish that you find in your bathroom or under your fridge is a hemimetabolous insect -
you may have spotted tiny ones only 1-2 mm in length which are actually young nymphs. Later nymphs that look very much
like the imago are often called juveniles. In these insects, the larval tissues gradually transform into the adult tissues.
In holometabolous insects, there are several instars of larvae (an exact number that varies with species) before the final instar
larva moults into the pupa stage. Each larva characteristically looks very different indeed from the adult, and they may even
look very different from one another. The pupa is characteristically immotile and inactive (though some are capable of
vigorous movements) and the classical example is that of the Lepidoptera (butterflies and moths) which is encased in a
chrysalis of secreted chitin or a cocoon of silk that the last instar larva spun around itself and is often found fastened to the
underside of a leaf upon which the larva fed. Flies also have characteristic pupae. If you leave a piece of meat outside for a
while, allowing carrion flies like blowflies to lay their eggs on it, and then place it in a box with some moist cotton wool to stop it
drying out, then you will observe the larvae (maggots, as fly larvae are called) hatch, feed and grow, turning the meat into a
liquid soup which the larvae ingest and finally turning into pupae when their food is exhausted. The pupae of these flies are
reddish cylindrical objects that are very rigid and tough an quite inert. Despite their apparent inertness, however, pupae are
metabolically very active, using up food reserves stored by the feeding larva. They use this energy to massively re-arrange
their tissues, which may liquefy at some stage, before reforming as adult tissues. This pupation process is a complete
metamorphosis and when ready the adult insect will emerge from the pupa, typically by gnawing a hole through it, pulling itself
out and allowing its wings to expand with blood and its cuticle to darken and harden. Butterflies are the best known examples
of this, changing from leaf-eating caterpillars into adult butterflies, but flies (Diptera) do this too, as do beetles (Coleoptera)
and Hymenoptera.
Having larvae that differ radically from the adult enables insects to exploit a variety of resources. Typically the larvae and the
adults (if they feed at all) consume different foods, thereby avoiding direct competition with one another and exploiting more
resources. A classic example is the dragonfly, whose larva lives in freshwater and is a ferocious predator that swims by jet
propulsion (expelling water from its anus) and catching tadpoles, small fish and other insects with its protrusible jaws
(reminiscent of the 'Alien' of sci-fi). The adults live mostly in the air, where they catch other insects to eat, mate on the wing
and even lay eggs whilst hovering, and only occasionally landing to rest.
The rove beetle Aleochara has a particularly strange life cycle. The adults are ferocious predators, eating fly maggots and the
like, whilst the larvae are parasitic (properly paraitoidic) on fly pupae. Each species of Aleochara is quite specific to the kind of
pupae its larvae will infect. Aleochara curtula infects the pupae of the carrion fly Calliphora and the adults will form societies
living on a single animal carcass. The dominant male will keep a harem of females on his carcass and ward off male rivals.
Some males, however, cheat by mimicing females (they smell the same as females and tend to be smaller than other males)
and mating with females in the harem when the dominant male isn't looking! The adults will eat maggots of the carrion flies that
feed on the carcass. Aleochara bipustulata and Aleochara bilineata (whose life-cycle is shown above) feed on certain
root-flies of the genus Delia, such as the onion-root fly, Delia antiqua, and the cabbage-root fly, Delia radicum. These flies lay
their eggs in the soil around their choice of plants (onions for Delia antiqua, cabbage, suede (rutabaga or yellow turnip) and
turnips for Delia radicum). The adult Aleochara also lay their eggs in the soil around these plants, especially plants that are
infected by the root-flies. These insects do not appear to be as social as Aleochara curtula, though they do tend to form
communal burrows around the infected plants and they eat root-fly maggots and other suitable items (but can be kept in the
laboratory by feeding them cat biscuits!).
Each adult female Aleochara bilineata lays about 10 eggs each day, and lives for 2-3 months, and so lays a total of about
500-600 eggs in her lifetime. The eggs hatch into first instar larvae. These larvae are reasonably well-developed with quite
powerful legs, strong jaws and well developed antennae and sensory bristles. They also have two rudimentary eyes, which
enable them to sense the direction light is coming from - they avoid light, burrowing down into the soil. They locate a root-fly
pupa in the soil and gnaw a hole in the pupal case, enter, seal the hole and then slowly devour the fly pupa developing within!
Insects exhibit two different principal types of life-cycle. Most insects lay eggs (though a few give birth to live young) which are
deposited in soil or attached to vegetation or some appropriate food source. In some insects, the egg hatches into a nymph
that resembles the adult, differing perhaps slightly in form, but much smaller; these are the hemimetabolous insects.
Holometabolous insects are those whose eggs hatch into a larva that differs greatly from the adult (imago) in form.
The tough exoskeleton (cuticle) of insects restricts their growth. To overcome this restriction, insects shed their skin every so
often, or moult. The number of moults is precise and adults do not moult. When an insect moults, its new skin is soft and white
at first, this is an insect's most vulnerable period. The insect can then expand itself like an inflatable toy by taking in air or
water. The new cuticle subsequently darkens and hardens, and the newly formed spaces, that resulted from inflation, are filled
with blood and are slowly replaced by tissues, until the next moult. We talk of instars, where the first form, which hatches from
the egg, is the first instar larva or nymph. This moults into the second instar larva or nymph, and so on, until the adult stage is
reached.
In hemimetabolous insects, the nymph undergoes a gradual metamorphosis into the adult form, appearing more like the adult
with each moult, but these changes are slight, since the first instar nymph resembled the adult anyway, and the most obvious
change is an increase in size. The silverfish that you find in your bathroom or under your fridge is a hemimetabolous insect -
you may have spotted tiny ones only 1-2 mm in length which are actually young nymphs. Later nymphs that look very much
like the imago are often called juveniles. In these insects, the larval tissues gradually transform into the adult tissues.
In holometabolous insects, there are several instars of larvae (an exact number that varies with species) before the final instar
larva moults into the pupa stage. Each larva characteristically looks very different indeed from the adult, and they may even
look very different from one another. The pupa is characteristically immotile and inactive (though some are capable of
vigorous movements) and the classical example is that of the Lepidoptera (butterflies and moths) which is encased in a
chrysalis of secreted chitin or a cocoon of silk that the last instar larva spun around itself and is often found fastened to the
underside of a leaf upon which the larva fed. Flies also have characteristic pupae. If you leave a piece of meat outside for a
while, allowing carrion flies like blowflies to lay their eggs on it, and then place it in a box with some moist cotton wool to stop it
drying out, then you will observe the larvae (maggots, as fly larvae are called) hatch, feed and grow, turning the meat into a
liquid soup which the larvae ingest and finally turning into pupae when their food is exhausted. The pupae of these flies are
reddish cylindrical objects that are very rigid and tough an quite inert. Despite their apparent inertness, however, pupae are
metabolically very active, using up food reserves stored by the feeding larva. They use this energy to massively re-arrange
their tissues, which may liquefy at some stage, before reforming as adult tissues. This pupation process is a complete
metamorphosis and when ready the adult insect will emerge from the pupa, typically by gnawing a hole through it, pulling itself
out and allowing its wings to expand with blood and its cuticle to darken and harden. Butterflies are the best known examples
of this, changing from leaf-eating caterpillars into adult butterflies, but flies (Diptera) do this too, as do beetles (Coleoptera)
and Hymenoptera.
Having larvae that differ radically from the adult enables insects to exploit a variety of resources. Typically the larvae and the
adults (if they feed at all) consume different foods, thereby avoiding direct competition with one another and exploiting more
resources. A classic example is the dragonfly, whose larva lives in freshwater and is a ferocious predator that swims by jet
propulsion (expelling water from its anus) and catching tadpoles, small fish and other insects with its protrusible jaws
(reminiscent of the 'Alien' of sci-fi). The adults live mostly in the air, where they catch other insects to eat, mate on the wing
and even lay eggs whilst hovering, and only occasionally landing to rest.
The rove beetle Aleochara has a particularly strange life cycle. The adults are ferocious predators, eating fly maggots and the
like, whilst the larvae are parasitic (properly paraitoidic) on fly pupae. Each species of Aleochara is quite specific to the kind of
pupae its larvae will infect. Aleochara curtula infects the pupae of the carrion fly Calliphora and the adults will form societies
living on a single animal carcass. The dominant male will keep a harem of females on his carcass and ward off male rivals.
Some males, however, cheat by mimicing females (they smell the same as females and tend to be smaller than other males)
and mating with females in the harem when the dominant male isn't looking! The adults will eat maggots of the carrion flies that
feed on the carcass. Aleochara bipustulata and Aleochara bilineata (whose life-cycle is shown above) feed on certain
root-flies of the genus Delia, such as the onion-root fly, Delia antiqua, and the cabbage-root fly, Delia radicum. These flies lay
their eggs in the soil around their choice of plants (onions for Delia antiqua, cabbage, suede (rutabaga or yellow turnip) and
turnips for Delia radicum). The adult Aleochara also lay their eggs in the soil around these plants, especially plants that are
infected by the root-flies. These insects do not appear to be as social as Aleochara curtula, though they do tend to form
communal burrows around the infected plants and they eat root-fly maggots and other suitable items (but can be kept in the
laboratory by feeding them cat biscuits!).
Each adult female Aleochara bilineata lays about 10 eggs each day, and lives for 2-3 months, and so lays a total of about
500-600 eggs in her lifetime. The eggs hatch into first instar larvae. These larvae are reasonably well-developed with quite
powerful legs, strong jaws and well developed antennae and sensory bristles. They also have two rudimentary eyes, which
enable them to sense the direction light is coming from - they avoid light, burrowing down into the soil. They locate a root-fly
pupa in the soil and gnaw a hole in the pupal case, enter, seal the hole and then slowly devour the fly pupa developing within!
| Insect Life Cycles |
Above: the first instar larva of Aleochara bilineata. Notice the small eyes, the sensory hairs that are especially
well-developed on the head and tail. Notice also the antennae, and the row of protuberances near the bottom margin of the
abdomen, each of these is surmounted by a spiracle pore for respiration. The legs are quite well developed and the
cuticle, especially in the head, is brown and quite rigid, which indicates that strong muscles attach to it. This individual has
been feeding, which is why its abdomen is quite full and bloated. This larva is free-living and the strong cuticle gives it then
it will die.
The larvae will try to avoid infecting a pupa that already contains another Aleochara larva (a process called
superparasitisation), unless pupae are in very short supply, in which case two or more larvae may enter the same pupa
and compete for resources, though usually only one will survive, it will be much reduced in size due its loss of food to its
rivals. If they are forced to parasitise an occupied pupa, then they prefer to select one that does not contain one of their
own siblings.
Whilst all this is going on, the fly pupa looks normal to a casual observer. The first instar larva will feed and then moult into
the second instar larva, which grows and moults into the third instar larva, which is the final larval stage. The second and
third instar larvae look identical, except that the third instar larva is much bigger. They have soft and white cuticles and
small legs - they don't need to move much and they don't need much protection against dehydration or predation, since
they are protected inside the tough pupal case of the fly. Their sensory bristles become much reduced. They lack the long
sensory hairs on the tail, which alert the first instar larva if anything tries to creep up on it. They lack the pair of eyes
present in the third-instar larva and their jaws are weaker, since they do not need to gnaw through the tough pupal case,
but simply feed on the soft liquefying tissues of the fly pupa. Their antenna are much reduced. They have what appears to
be an ocellus, a type of eye, in the middle, on top of the head. Ocelli form poor images and serve only to monitor light
levels. They help insects determine day length and the time of year. The Aleochara larvae will overwinter inside the host fly
pupa if necessary, before completing the life-cycle.
well-developed on the head and tail. Notice also the antennae, and the row of protuberances near the bottom margin of the
abdomen, each of these is surmounted by a spiracle pore for respiration. The legs are quite well developed and the
cuticle, especially in the head, is brown and quite rigid, which indicates that strong muscles attach to it. This individual has
been feeding, which is why its abdomen is quite full and bloated. This larva is free-living and the strong cuticle gives it then
it will die.
The larvae will try to avoid infecting a pupa that already contains another Aleochara larva (a process called
superparasitisation), unless pupae are in very short supply, in which case two or more larvae may enter the same pupa
and compete for resources, though usually only one will survive, it will be much reduced in size due its loss of food to its
rivals. If they are forced to parasitise an occupied pupa, then they prefer to select one that does not contain one of their
own siblings.
Whilst all this is going on, the fly pupa looks normal to a casual observer. The first instar larva will feed and then moult into
the second instar larva, which grows and moults into the third instar larva, which is the final larval stage. The second and
third instar larvae look identical, except that the third instar larva is much bigger. They have soft and white cuticles and
small legs - they don't need to move much and they don't need much protection against dehydration or predation, since
they are protected inside the tough pupal case of the fly. Their sensory bristles become much reduced. They lack the long
sensory hairs on the tail, which alert the first instar larva if anything tries to creep up on it. They lack the pair of eyes
present in the third-instar larva and their jaws are weaker, since they do not need to gnaw through the tough pupal case,
but simply feed on the soft liquefying tissues of the fly pupa. Their antenna are much reduced. They have what appears to
be an ocellus, a type of eye, in the middle, on top of the head. Ocelli form poor images and serve only to monitor light
levels. They help insects determine day length and the time of year. The Aleochara larvae will overwinter inside the host fly
pupa if necessary, before completing the life-cycle.
Above and below: the second instar larva of Aleochara. The cuticle is soft and white and the powers of locomotion and
sensors are much reduced. The body is bloated with food. Some spiracles are visible (the others were hidden by 'crud'
since these insects are messy eaters, and practically swim in the liquid tissues of their host). This reduction in systems is
common in parasitic creatures and is called degeneration.
sensors are much reduced. The body is bloated with food. Some spiracles are visible (the others were hidden by 'crud'
since these insects are messy eaters, and practically swim in the liquid tissues of their host). This reduction in systems is
common in parasitic creatures and is called degeneration.
The third instar larva transforms into the pupa. The pupa is soft and white and ensheathed in a soft, transparent
membrane. It has no need to form a tough cocoon, chrysalis or pupal case, since it is still enclosed in the host fly
puparium (pupal case). When ready it will emerge by gnawing a hole in the pupal case and escaping as an adult
beetle - so, instead of the usual fly emerging from the pupa, an alien beetle will emerge instead!
Although the adults are predators and continue to feed on maggots, Aleochara needs to consume only one prey
insect to complete its life-cycle, and it will consume one insect and consume it entirely. The larva is therefore
described as a parasitoid. A parasitoid is a special kind of parasite. A parasite lives on or in a host and feed upon it,
but often without killing its host, indeed a parasite benefits by NOT killing its host. Predators benefit by completely
destroying and consuming multiple prey items. A parasitoid is half-way between a predator and a parasite - it needs
to kill and consume a single host to complete its development.
Parasitoids have featured famously in science fiction. One of the first to do so (if not the first) was the Wirrrn - a larger
than man-sized alien insectoid creature that infected humans that slept in suspended animation on board the Nova
Beacon space ark in Dr Who. Upon waking they found one of their crew missing and a strange presence growing on
board the space station! The alien from the film of the same name was also a parasitoid. These parasitoids fed on
humans, on Earth they don't do this, but to a fly, the horror is real!
One question remains - how does Aleochara locate its host?
The first part of this question concerns how the adults locate onion and cabbage plants, but we shall look at that in a
section on insect behaviour. Here we shall look at how the first instar larva locates a fly pupa. Studies in the literature
suggest that the larva executes a random search and recognises a suitable host by touch when it bumps into it by
chance. However, below we present an alternative model. First, we shall look what sensors the first instar larva has on
its antennae. a diagram of the antenna is shown below:
membrane. It has no need to form a tough cocoon, chrysalis or pupal case, since it is still enclosed in the host fly
puparium (pupal case). When ready it will emerge by gnawing a hole in the pupal case and escaping as an adult
beetle - so, instead of the usual fly emerging from the pupa, an alien beetle will emerge instead!
Although the adults are predators and continue to feed on maggots, Aleochara needs to consume only one prey
insect to complete its life-cycle, and it will consume one insect and consume it entirely. The larva is therefore
described as a parasitoid. A parasitoid is a special kind of parasite. A parasite lives on or in a host and feed upon it,
but often without killing its host, indeed a parasite benefits by NOT killing its host. Predators benefit by completely
destroying and consuming multiple prey items. A parasitoid is half-way between a predator and a parasite - it needs
to kill and consume a single host to complete its development.
Parasitoids have featured famously in science fiction. One of the first to do so (if not the first) was the Wirrrn - a larger
than man-sized alien insectoid creature that infected humans that slept in suspended animation on board the Nova
Beacon space ark in Dr Who. Upon waking they found one of their crew missing and a strange presence growing on
board the space station! The alien from the film of the same name was also a parasitoid. These parasitoids fed on
humans, on Earth they don't do this, but to a fly, the horror is real!
One question remains - how does Aleochara locate its host?
The first part of this question concerns how the adults locate onion and cabbage plants, but we shall look at that in a
section on insect behaviour. Here we shall look at how the first instar larva locates a fly pupa. Studies in the literature
suggest that the larva executes a random search and recognises a suitable host by touch when it bumps into it by
chance. However, below we present an alternative model. First, we shall look what sensors the first instar larva has on
its antennae. a diagram of the antenna is shown below:
and 3) and two segments that may be true segments or simply outgrowths and so are labelled as pseudosegments (PS1
and PS2). Segment 3 forms what is called the outer lobe (OL) and a cone forms the inner lobe (IL). Segment 2 bears two
hairs that are evidently touch mechanoreceptors with the characteristic structure of trichoid sensilla with a dendrite
attached to flexible sockets in the base and poreless hairs with no dendrites in their lumens. The outer lobe bears three
large hairs (about 60 micrometres long) that are very long in proportion to the antenna. These hairs have been assumed
to have only a tactile role, but they have the structure of dual mechano-gustatory receptors, with a dendrite in the flexible
socket and one or two sheathed dendrites running the length of the hair inside the hair lumen. It is not known whether
these hairs end in a terminal pore, but there are no pores along the length of the hairs and the tips can appear bulbous in
scanning electron microscopy, perhaps due to exudate from a terminal pore or due to some associated structure. These
hairs may be used to probe potential host pupae and may pick up sensory cues indicating whether or not the pupa is
already infected, and if so, whether or not it is infected by close kin. The outer lobe also bears three small trichoid sensilla
(S) which also contain dendrites and so appear to be dual mechano-gustatory receptors, and a blunt peg (P).
The peg (P) and the inner lobe are grooved. There is no detailed information regarding the structure of the small peg, but
the inner lobe has the structure characteristic of a large and complex olfactory sensor, strikingly similar to those on the
antennae of larva of the beetle Ctenicora destructor and the housefly Musca domestica. These organs are grooved, with
pores running along the grooves. The pores lead into a large lymph filled cavity with an outer cortex of dendritic branches
emanating from multiple primary dendrites. The inner lobe has a volume of about 1000 cubic micrometres and is much
larger (by about 350-fold in volume) and more complex than any olfactory organ found on the adult antenna, although the
adult's antenna has many more (about 1500) smaller olfactory sensilla. Thus, it appears that the larva has a
well-developed sense of smell, but what does it use it for? One clue comes from examining the antenna of the second
instar larva, shown below:
and PS2). Segment 3 forms what is called the outer lobe (OL) and a cone forms the inner lobe (IL). Segment 2 bears two
hairs that are evidently touch mechanoreceptors with the characteristic structure of trichoid sensilla with a dendrite
attached to flexible sockets in the base and poreless hairs with no dendrites in their lumens. The outer lobe bears three
large hairs (about 60 micrometres long) that are very long in proportion to the antenna. These hairs have been assumed
to have only a tactile role, but they have the structure of dual mechano-gustatory receptors, with a dendrite in the flexible
socket and one or two sheathed dendrites running the length of the hair inside the hair lumen. It is not known whether
these hairs end in a terminal pore, but there are no pores along the length of the hairs and the tips can appear bulbous in
scanning electron microscopy, perhaps due to exudate from a terminal pore or due to some associated structure. These
hairs may be used to probe potential host pupae and may pick up sensory cues indicating whether or not the pupa is
already infected, and if so, whether or not it is infected by close kin. The outer lobe also bears three small trichoid sensilla
(S) which also contain dendrites and so appear to be dual mechano-gustatory receptors, and a blunt peg (P).
The peg (P) and the inner lobe are grooved. There is no detailed information regarding the structure of the small peg, but
the inner lobe has the structure characteristic of a large and complex olfactory sensor, strikingly similar to those on the
antennae of larva of the beetle Ctenicora destructor and the housefly Musca domestica. These organs are grooved, with
pores running along the grooves. The pores lead into a large lymph filled cavity with an outer cortex of dendritic branches
emanating from multiple primary dendrites. The inner lobe has a volume of about 1000 cubic micrometres and is much
larger (by about 350-fold in volume) and more complex than any olfactory organ found on the adult antenna, although the
adult's antenna has many more (about 1500) smaller olfactory sensilla. Thus, it appears that the larva has a
well-developed sense of smell, but what does it use it for? One clue comes from examining the antenna of the second
instar larva, shown below:
Above: the antenna of the 2nd-instar larva of Aleochara bilineata. The scale bar is 20 micrometres. Notice that the three
segments remain, but that the sensors are all much reduced. The trichoid sensilla are shorter, with the three longest
reduced to about 11 micrometres in length (a 5.5-fold reduction in length) and the three shorter sensilla reduced 2.5-fold
to 4 micrometres in length. The olfactory inner lobe is also much reduced to about 40% of the volume of that in the first
instar. This reduction indicates that the functional requirements of these sensors are reduced once the larva is inside its
host pupa.
Thus we can conclude that chemoreceptors, including olfactory sensors, as well as touch sensors are very important to
the first instar larva. Olfactory sensors may be used to avoid predators, or to ensure that the insect does not stray too far
from the host plant, or they may be used to sniff-out host pupae - we don't know! They are not used to locate food other
than a host pupa, since the larvae do not feed until they enter their host. There presence, however, forces us to rethink
the accepted wisdom that the larvae locate hosts purely by random searching and verify their suitability by touch alone.
Why then do the larvae appear to make random search movements? This may be an artefact of the experiment. It is not
realistic to place pupae in fresh sand or soil and then add larvae and expect them to locate the pupae in the same way
that they would do so in nature, simply because as the root-fly maggots get ready to pupate, they leave the vicinity of the
roots and burrow down deeper into the soil away from the plant. It would, therefore, be expected that the larvae would
leave a trail in the soil and maybe the Aleochara larvae can sniff-out the odours in this trail? Removing the pupae and
transplanting them will abolish such trail cues and then the larvae may have to resort to random searching. More research
is needed to resolve these issues.
segments remain, but that the sensors are all much reduced. The trichoid sensilla are shorter, with the three longest
reduced to about 11 micrometres in length (a 5.5-fold reduction in length) and the three shorter sensilla reduced 2.5-fold
to 4 micrometres in length. The olfactory inner lobe is also much reduced to about 40% of the volume of that in the first
instar. This reduction indicates that the functional requirements of these sensors are reduced once the larva is inside its
host pupa.
Thus we can conclude that chemoreceptors, including olfactory sensors, as well as touch sensors are very important to
the first instar larva. Olfactory sensors may be used to avoid predators, or to ensure that the insect does not stray too far
from the host plant, or they may be used to sniff-out host pupae - we don't know! They are not used to locate food other
than a host pupa, since the larvae do not feed until they enter their host. There presence, however, forces us to rethink
the accepted wisdom that the larvae locate hosts purely by random searching and verify their suitability by touch alone.
Why then do the larvae appear to make random search movements? This may be an artefact of the experiment. It is not
realistic to place pupae in fresh sand or soil and then add larvae and expect them to locate the pupae in the same way
that they would do so in nature, simply because as the root-fly maggots get ready to pupate, they leave the vicinity of the
roots and burrow down deeper into the soil away from the plant. It would, therefore, be expected that the larvae would
leave a trail in the soil and maybe the Aleochara larvae can sniff-out the odours in this trail? Removing the pupae and
transplanting them will abolish such trail cues and then the larvae may have to resort to random searching. More research
is needed to resolve these issues.
Above: a longitudinal section through the inner lobe of the 1st-instar larva of Aleochara bilineata. (Unpublished data,
courtesy of C. Skilbeck, J. C. K. Brown and M. Anderson, 1996). Note the pores (P) in the bottom of the grooves and the
cuticular struts (C) visible as this section is just beneath the surface. Also note the extracellular flat membranous
vesicles, historically called dictyosomes, but since this descriptive term is now specifically to certain Golgi bodies, it is
better to call these extracellular membranous discs. The scale bar is 500 nanometres.
courtesy of C. Skilbeck, J. C. K. Brown and M. Anderson, 1996). Note the pores (P) in the bottom of the grooves and the
cuticular struts (C) visible as this section is just beneath the surface. Also note the extracellular flat membranous
vesicles, historically called dictyosomes, but since this descriptive term is now specifically to certain Golgi bodies, it is
better to call these extracellular membranous discs. The scale bar is 500 nanometres.
Above: longitudinal sections through the inner lobe, which are more medial (deeper into the structure). Top: a
dendritic branch with a swelling (S) can be seen just beneath the surface of the cuticle (such beaded dendrites are
characteristic of these types of sense organs in insect larvae). Note the cuticular struts (C), with pores in-between,
and bundles of darkly staining lipoidal vesicles (V). Scale bar = 500 nm. Bottom: a large dendrite (D) gives rise to
nanometres. (Unpublished data, courtesy of C. Skilbeck, J. C. K. Brown and M. Anderson, 1996).
dendritic branch with a swelling (S) can be seen just beneath the surface of the cuticle (such beaded dendrites are
characteristic of these types of sense organs in insect larvae). Note the cuticular struts (C), with pores in-between,
and bundles of darkly staining lipoidal vesicles (V). Scale bar = 500 nm. Bottom: a large dendrite (D) gives rise to
nanometres. (Unpublished data, courtesy of C. Skilbeck, J. C. K. Brown and M. Anderson, 1996).
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