Diversification of gut morphology in caterpillars is associated with defensive behavior

Abstract

SUMMARY The relationship between insect gut structure and foraging strategy has been studied for several hundred years; however, we know little about how, or even if, other common insect behaviors are linked to gut morphology. For example, many insects defend themselves by regurgitation, a behavior which is expected to be closely connected to gut structure. Caterpillars belong to an insect taxon, the Lepidoptera, with a particularly well-studied digestive tract and a known predilection for defensive regurgitation. I have explored whether defensive regurgitation is associated with specific gut structure by examining the relationship between defensive behavior and gut morphology in the larvae of 36 butterfly and moth species. My analysis shows that predilection to defensively regurgitate is closely associated with fore- and midgut morphology. Species that primarily regurgitate in defense have enlarged foreguts and shortened midguts, whereas the opposite relationship is seen among those that do not readily regurgitate. These results reveal that: (i)defensive regurgitation is not the primary defense of all caterpillars, and(ii) gut morphology can be associated with other factors than the chemical and mechanical properties of ingested foods. These findings challenge long held beliefs concerning the ubiquity of defensive regurgitation in caterpillars and the causes of diversification of gut morphology, and further reinforce the gut's standing as a complex and sophisticated organ.

Insects had it first: surfactants as a defence against predators

“…Many herbivorous insects such as grasshoppers or the larval stages of sawflies and Lepidoptera regurgitate their gut contents when disturbed (Grant 2006). Numerous studies suggest this behaviour to be an effective defence mechanism because the ejected oral secretion (OS) may have deterrent effects against vertebrate and invertebrate predators such as lizards, birds or ants (Eisner 1970;Peterson et al 1987;Codella & Raffa 1995;Sword 2001).…”

Larvae of the fall webworm, Hyphantria cunea, inhibit cyanogenesis in Prunus serotina

“…The foregut of the webworm constitutes 49% of the total length of the alimentary tract, proportionately the longest of any of 33 species of caterpillars measured by Grant (Grant, 2006); mean=19%. By contrast, the foregut of the eastern tent caterpillar constitutes 21% of the total gut length (Grant, 2006). Following a single bout of feeding, the foregut of the tent caterpillar is fully packed with leaf fragments to the extent that the bolus strains its capacity (Snodgrass, 1922).…”

Whistling in caterpillars (Amorpha juglandis, Bombycoidea): sound-producing mechanism and function

“…Attacks with blunt forceps were conducted to simulate an attack by a bird or the bite of a predatory insect (e.g. Bowers, 2003;Grant, 2006). Ten trials were performed where the resting caterpillar was attacked once near the head or abdomen and then quickly released.…”

“…Grant (Grant, 2006) categorizes caterpillars according to how much they regurgitate in response to attacks with forceps: primary regurgitators regurgitate soon after attack (1-3 pinches), can control the amount released and the direction it is aimed and re-imbibe the bubble when attacks cease. Secondary regurgitators (4-6 pinches) will regurgitate after thrashing, biting or trying to escape, cannot control the amount released and do not re-imbibe.…”

“…These 'secondary' defenses can be chemical, physical or behavioural, including the presentation of traits that mimic unpalatable or potentially dangerous animals. Chemical defenses can be systemic, or emitted through regurgitation (Grant, 2006). In several cases, these are advertised visually through bright coloration.…”