Instar larvae of the saddleback caterpillar, Acharia stimulea. My friend Martha brushed one of these today. FIRE!!!
Acharia stimulea (Clemens) is a limacodid moth, or slug moth, best known for its larval growth phase. Distinct bright color patterns and the presence of venomous, urticating spines lead to its recognition as the saddleback caterpillar. It is native to a large range in the eastern United States and able to feed on a wide array of host plant species. This species can survive well in northern temperate areas and warmer southern climates. The saddleback caterpillar is encountered most frequently as a medically significant pest, and has minor effects in landscaping and agriculture.
The family Limacodidae is recognized for its members having fleshy horns on the body, each containing numerous hollow spines. In Acharia stimulea, these hollow spines are present on the edges of the body length, but are concentrated on tuburculated protrusions that adorn both ends of the larvae creating a total of four large horns. Spinose processes vary in shape, length and articulation relative to where they are on the body, and whether or not multiple types are in proximity to each other (Dyar and Morton 1986, Gilmer 1925, Packard 1983). All of them are capable of breaking away from the body and remaining embedded in contacted surfaces. These spines contain hemolytic and vesicating venom, secreted from nearby glands, that cause direct tissue damage (Scott 1963, Edwards et al. 1986, Murphy et al. 2009).
Saddleback larvae use their great adhesive properties to stay firmly mounted to a surface. When coupled with the venomous spines on the body, this allows the caterpillar to remain attached to any selected surface and offer its spines to aggressors. Sometimes, if disturbed, larvae will arch their back inward in an attempt to create contact with all anterior and posterior horns (Packard 1893, Murphy et al. 2009); however, they are unable to retract the head capsule into the body for protection (Packard 1893).
The aposematic signaling and venomous spines function together to preserve the caterpillar, as the metabolic cost of developing either characteristic is steep. By having both, the organism can enhance the effects of both characteristics and further decrease the probability of aggressive contact (Speed and Ruxton 2005). The large, potent spines of Acharia stimulea have been shown to effectively deter vertebrate and invertebrate aggressors, with specific interactions showing reduviid bugs, chrysopid lacewings, and vespid wasps avoiding the saddleback caterpillars rather than attacking them (Murphy et al. 2009).
Photograph by Barefoot Skinny
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