Externalizing Allomones By Reflex Bleeding

Many insect species, particularly beetles, externalize their distinctive defensive compounds in a blood carrier rather than discharging them as components in an exocrine secretion.

Cantharidin, the terpenoid anhydride synthesized by adult beetles in the families Meloidae and Oedemeridae, is externalized in blood discharged reflexively from the femorotibial joints. The repellent properties of cantharidin were established more than 100 years ago, and the ability of amphibians to feed on these beetles with impunity has been long known, as well. Cantharidin possesses a wide spectrum of activities, including inducing priapism in the human male, and it has been reported to cause remission of epidermal cancer in mammals. Although its role as a repellent and lesion producer certainly documents its efficacy as a predator deterrent, its potent antifungal activity may be of particularly great adaptiveness in protecting developing meloid embryos from entomopathogenic fungi present in their moist environment.

Autohemorrhage, from the femorotibial joints, is widespread in many species of ladybird beetles (Coccinellidae), most of which are aposematic. The blood is generally fortified with novel alkaloids that are outstanding repellents and emetics (i.e., inducers of vomiting) as well.

Adult fireflies (Photinus spp.) produce novel steroids (lucibufagins) that are effective repellents and inducers of emesis in invertebrates and vertebrates. Reflex bleeding from specialized weak spots in the cuticle along the elytra and antennal sockets externalizes these steroids.

Sometimes, rapidly coagulating blood, free of allomones, is used defensively.

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