14.4.4. Organs of chemical defense
Endogenous defensive chemicals (those synthesized within the insect) generally are produced in specific glands and stored in a reservoir (Box 14.4). Release is through muscular pressure or by evaginating the organ, rather like turning the fingers of a glove insideout. The Coleoptera have developed a wide range of glands, many eversible, that produce and deliver defensive chemicals. Many Lepidoptera use urticating (itching) hairs and spines to inject venomous chemicals into a predator. Venom injection by social insects is covered in section 14.6.
In contrast to these endogenous chemicals, exogenous toxins, derived from external sources such as foods, are usually incorporated in the tissues or the hemolymph. This makes the complete prey unpalatable, but requires the predator to test at close range in order to learn, in contrast to the distant effects of many endogenous compounds. However, the larvae of some swallowtail butterflies (Papilionidae) that feed upon distasteful food plants concentrate the toxins and secrete them into a thoracic pouch called an osmeterium, which is everted if the larvae are touched. The color of the osmeterium often is aposematic and reinforces the deterrent effect on a predator (Fig. 14.6). Larval sawflies (Hymenoptera: Pergidae), colloquially called “spitfires”, store eucalypt oils, derived from the leaves that they eat, within a diverticulum of their fore gut and ooze this strong-smelling, distasteful fluid from their mouths when disturbed (Fig. 14.7).
Eversion of this glistening, bifid organ occurs when the larva is disturbed and is accompanied by a pungent smell.
When disturbed, the larvae bend their abdomens in the air and exude droplets of sequestered eucalypt oil from their mouths.