Chemical and Physical Fecal Defense Mechanisms in Insects
Biology 342 Fall 08
Instead of discarding feces, some insects save their waste matter for defensive purposes such as frass chains and fecal shields. Frass chains are stick-like structures made of feces and silk. Larvae rest on these chains, suspended away from the leaf, when not feeding. Fecal shields are coverings over the back of a larvae made of feces. They can be physical and also chemical barriers to predation.
Fig. 6. Eunica bechina larvae on frass chain. (Photo from Freitas and Oliveira 2001).
Frass chains are assembled by larva from feces and silk. They are located on the underside of leaves that the larva feeds on (Fig. 1) (Machado and Freitas 2001) . An alternate form of frass chain construction has been observed in the larvae of Anaea ryphea, a Nymphalid. These larvae feed on either side of the main vein of a leaf, starting at the tip of the leaf. Once the main vein has been exposed by the larvae feeding on either side of it, the exposed tip of the vein is used as a resting place. The larvae will construct a frass chain along this vein. It has been hypothesized that the frass chain strengthen or extends the vein, making it a better resting place (Caldas 1994).
There is currently no evidence that there is a significant chemical component to the defensive characteristics of the frass chain.
Fig. 7 (right). Plagiometriona clavata larva with fecal shield. (Photo copyright © 2005 Tam Stuart).
Beetle larvae can push their feces onto their backs from their dorsal anus.The shields can form clumped frass piles, such as that of the Blepharida rhois (Fig. 3), or umbrella shaped coverings such as that of the Plagiometriona clavata (Fig. 2) (Vencl and Morton, 1998).
The shields of the Plagiometriona clavata beetle are chemical deterrents, rather than repellents. The deterrence can be observed by generalist predators, such as the ant Formica subsericea, who will make contact with a shield, but then back away and clean itself instead of proceeding in an attack (Vencl and Morton 1998). Chemical repellents would comparatively keep predators from even touching the shield. The deterrent in the feces comes from the beetle’s food source of Solanum sulmara. The beetle eats the leaf, but does not metabolize such molecules as steroidal glycoalkoloid derivatives, dioscin metabolites (from the plant’s saponins), fatty acids, and phytol derivatives that all result in predator deterrence. In addition to these chemicals, Vencl et al. also found that the fecal shield is not a physical barrier, as the shield without the chemical dererrents will not stop predation of the larvae (Vencl et. al. 1999).