Larva : Cannibals, Carnivores AND Myrmecophiles

Cannibals

Larvae often find themselves on plants with insufficient foliage to sustain them, so at these times they naturally tend to wander in search of other plants. Often it can take a considerable time for a larva to locate another specimen of it’s foodplant, and starvation sets in.

It is normal for a starved larva to nibble intensely at any object it encounters. If the object happens to be another larva it is likely that its skin would be punctured, and the mouthparts of the attacker would come into contact with its body fluids. The fluids have a similar chemical content and taste to the foodplants of the larva, and hence cannibalism arose.

Cannibalism has two possible advantages for larvae – firstly by eradicating competitors they ensure they have enough leaves for themselves. Secondly, captive rearing has shown that cannibalistic larvae grow faster than non-cannibals, probably because they save themselves the time-consuming business of digesting and processing the vegetation.

The larvae of many species including the Orange tip Anthocharis cardamines and the Dun-bar moth Cosmia trapezina are cannibalistic. They normally feed on foliage but will attack and eat any other caterpillar that they encounter. Most members of the Lycaenidae also have very strong cannibalistic tendencies, and many are carnivorous on aphids or ant grubs.

Carnivores

The larva of the Large Blue Maculinea arion feeds in its early instars on the flowers and leaves of Thymus pulegioides, but is aggressively cannibalistic towards other larvae. Upon reaching the 4th instar it loses all interest in feeding, releases it’s grip on the foodplant and falls to the ground. It wanders about until it is located by a Myrmica sabuleti ant. The ant uses it’s antennae to caress the larva, stimulating it to secrete a honey-like fluid from the Newcomer’s gland on it’s back. After a while the larva taps its posterior segments against the ground, sending a vibratory signal to the ant. The ant then wanders off but returns later with other ants to further “milk” the larva.

After several milking sessions the larva becomes immobile and hunches it’s back. This allows the ant to seize it and carry it into the brood chamber of the ant nest. Once settled in its new home the larva becomes carnivorous, feeding on tiny ant grubs. Only the largest ant nests produce the 1500 or so grubs that are necessary for the Large Blue larva to complete its growth.

The larva is tolerated and protected by the adult ants in exchange for providing them with a regular supply of “honey” from it’s dorsal gland. It also emits pheromones and produces sounds that have been shown to appease the ants and assure its safety. The arion larvae spend several weeks in the ant nest, feeding exclusively on ant grubs until they pupate.

In Africa the larvae of Lepidochrysops have a similar association with Camponotus ants. Another Lycaenid species Euliphyra hewitsoni lives in the nests of Oecophylla tailor-ants, also feeding on a diet of ant grubs. Its cousin the Woolly Leg butterfly Lachnocnema lays its eggs amidst colonies of psyllid or membracid bugs. The resulting larvae feed carnivorously throughout their lives, attacking and devouring the tiny insects.

In Borneo, Allotinus apries feeds on coccids when it is small. As it grows bigger the larva develops protrusions on its body. These protrusions are used as grapples by an ant called Myrmecaria lutea which carries the larva to it’s nest. Thereafter the larva lives within the nest, feeding on ant grubs.

The extraordinary caterpillar of Liphyra brassolis lives inside the nests of weaver ants Oecophylla smaragdina, devouring hundreds of ant grubs. A single nest can house as many as 5 or 6 Liphyra larvae. Any other larva that found it’s way into an ant nest would be killed, but the tortoise-like Liphyra larva has a built-in survival kit in the form of a tough chitinous carapace that is impervious to ant bites. The ants repeatedly attempt to flip it over to reach the soft under-belly, but the larva uses it’s powerful sucker-like feet to pull the carapace down and seal it limpet-fashion against the substrate, defeating all attempts by the ants to gain entry.

Periodically the larva lifts the carapace slightly and pops it’s head out to snatch an ant grub with its mandibles. In an instant the grub is pulled under the carapace. The larva then pierces the skin of the grub, and sucks out the juices. The grub’s empty skin is then ejected, and the killer larva moves on to find its next victim.

The pupa of Liphyra brassolis is formed within the ant nest. The ants don’t attack the pupa as it is able to mollify them by using chemical deterrents, and by stridulating. Research on Lycaenids has demonstrated that the larvae and pupae of at least 150 species worldwide are able to generate a “melody” of audible chirps that appease ants.

Liphyra brassolis larva, Siem Reap, Cambodia – Dani Jump

By associating with ants, Lycaenid larvae gain protection from other small predatory insects which avoid ants in case they are attacked. For the same reason larvae are less likely to be attacked by parasitoids – one study found for example that ant-attended larvae of Glaucopsyche suffer much lower levels of parasitism by Braconid wasps and Tachinid flies compared with unattended larvae.

All documented Lycaenid larvae have one or more adaptations specialised for association with ants. They include the dorsal Newcomers organ which lies between the 7th/8th abdominal segments; and a pair of eversible tentacles either side of it. These secrete a substance similar to aphid honeydew, which acts as an ant-attractant. The substance contains glucose, sucrose, fructose and amino acids including serine, histidine, glutamic acid, lysine and arganine. The variable proportions of the amino acids gives the secretion of each species a distinctive “signature” which ensures that it attracts the correct species of ant.

Another adaptation of Lycaenid larvae is the presence of epidermal glands ( pore cupolas ) which exude substances that appease the normally aggressive ants. In combination these adaptations allow the larvae to manipulate the behaviour of ants for their own benefit.

Species that have dependent, mutually beneficial or symbiotic relationships with ants are known as myrmecophiles. The act of feeding on ants is called myrmecophagy. The act of feeding on aphids or other homopteran bugs is called aphytophagy.

The Evolution of Myrmecophagy and Aphytophagy

Myrmecophagy and aphytophagy probably have their origins in long-term exposure to ant-attended homopterans. The following hypothetical example might illustrate how such behaviours evolved:The larva of the Purple Hairstreak Quercusia quercus feeds on oak leaves. Ants forage on oaks to obtain honeydew, which they milk from aphids and from Quercusia larvae.

It is feasible that ants might at some stage in the future acquire the habit of carrying submissive quercusia larvae to their nests. There the larva might feed on substances regurgitated by ants, which are chemically similar to the oak sap that aphids convert into honeydew. Ants feed their grubs on these regurgitations, so the flesh of the grubs probably has a taste similar to oak leaves. A hungry Quercusia larva, which like most other Lycaenidae has cannibalistic tendencies, would soon start nibbling at objects in the ant nest, and it would not be long before it started to eat ant grubs.

In order for larvae to be able to co-exist with ants they had to evolve ways to protect themselves from attack. Lycaenid larvae for example have skin that is about 5 times thicker than that of those in other families of Lepidoptera, and is thus more resistant to ant bites. Another adaptation is the evolution of tough fleshy lappets along the lower edge of the caterpillar’s abdomen. These enable it to seal its body against the substrate, making it harder for an ant to flip it over to reach the soft underbelly. Ants have acute eyesight and tend to run towards moving objects, so Lycaenid larvae make themselves less obvious by adopting a slow gliding form of locomotion.

Lycaenid larval feeding

The table below lists the larval feeding behaviour of several genera of Lycaenidae. The subfamilies Lipteninae, Poritiinae and Lycaeninae have larvae which exclusively on plants. Larvae of Curetinae are plant-feeders and are attended by ants although there is no evidence yet of ants milking them. The Riodinidae, which some workers consider to be a subfamily of Lycaenidae, includes many species that are suspected of being carnivorous or aphytophagous.

More odd feeding habits

The moth Ceratophaga vicinella lays it’s eggs on the empty shells of dead Florida Gopher Tortoises Gopherus polyphemus. The resulting larvae feed gregariously on the keratin shells, constructing a mass of silk tubes which act as anchors, connecting the outer shell to the sandy substrate. Many moths have larvae that feed on dung.

These include Acrolophus pholetus and Idia gopheri, which both spend their lives feeding on dung within the burrows of the Florida Gopher Tortoise.

In Africa, the caterpillars of other Ceratophaga species feed in tunnels within the hooves and horns of antelopes and cattle. Ceratophaga species are members of the Tineidae – the family to which clothes moths belong. The larvae of the Case-bearing Clothes moth Tinea pellionella are notorious for their habit of eating holes in woollen fabrics but in the wild state live within bird’s nests, feeding on the keratin in hairs and feathers.

The larvae of the related Tapestry moth Trichophaga tapetzella feed on coarser fibres, and are also common in owl pellets ( regurgitated fur and bone ).