The Puss Caterpillar’s Verrucae
The primary stinging apparatus of the puss caterpillar is vested in verrucae (warty projections adorned with sharp spines under a coat of ornamental hairs) that cover the entire dorsal body of the caterpillar. In the photograph below, we can see each individual verruca that punctuates the periphery of the animal’s ventral body. This particular caterpillar–obtained during a study I conducted in October and November of 2009 to identify the puss caterpillar’s natural predators–has been parasitized by a fly in the Tachinidae family; the parasitoid maggot(s) inside the caterpillar induce systemic changes that manifest in various ways. In this case the parasitoid larvae appear to induce an infection that inflames the anatomical structures leading to and from the verrucae, causing them to turn purplish-black, so they contrast sharply with the pale, surrounding tissues.
Infected Puss Caterpillar Ventral Surface
Infected Verrucae
The purplish structures attached to each verruca demonstrate just how important the stinging apparatus is to the caterpillar. Each verruca is supplied with neural connections that enable the caterpillar to energize them on command, and with extensive vascular ducts that ensure constant nourishment from the caterpillar’s hemocoel.
A micrograph of a dorsal verruca from another puss caterpillar (also from the October-November 2009 study) is shown below. In order to separate two specimens for study, I had to fracture the pupal covering of a caterpillar that had recently pupated, exposing the body surface after most of its ornamental hairs had been exfoliated to produce the pupa. This left the venomous spines exposed. Each spine (consistent with the findings of N. C. Foot, in his 1921 paper) measures from 0.3 to 1mm in length, and 10 to 45 microns in diameter. Notice that each spine is hollow, a fact that is confirmed by the observation of bubbles within the venom they contain (see, for example, the third spine from the left, and the bubble that shows as a light spot in its shaft, not far from its sharp, pointed tip). Notice also, as described by Foot, that each spine emerges from a larger, easily discernible bulb, at its base. Foot states that each bulb connects to its spine with a chitinous diaphragm. This diaphragm provides a weak spot where the spine, on penetrating the tissues of a predator, will tend to break off. For this reason, a person who has been stung by one of these caterpillars should expect that, even though they won’t be visible to the naked eye, the sting site will contain numerous, microscopic spines that project from the skin, and that measure from microns to as much as 1mm in length. Removing these spines manually, using adhesive tape, is an important method of mitigating the severity and duration of the envenomation event.
Puss Caterpillar Verrucae
The importance of removing the spines as quickly as possible cannot be overemphasized. According to some authorities, the venom within each spine is transmitted to the tissues in which it is embedded by at least two routes: first by capillary action at both ends of the spine (the proximal end, opened by the action of snapping it off the bulb that attaches it to the verruca, and the distal end, which opens when the resinous plug at that end is dissolved by constituents in the tissues the spine is embedded in), as well as at at intermediate fractures that occur during or after penetration, and second by osmotic action, as the venom seeps through the cuticle of the spine–which has been made permeable by immersion in the body fluids of the sting site–into the adjacent tissues.
Besides the verrucae, it is apparent that structures in the caterpillar’s feet are capable of inflicting painful stings as well. From examination of the sting wounds experienced by persons who have contacted the puss caterpillar’s ventral surface–principally by having, or allowing, the caterpillar to travel across the unprotected skin, we notice red vesicles at each location where the animal’s feet touch the skin at the moment the caterpillar is led to sting the victim.
Thoracic legs & Prolegs
In addition to the stinging apparatus in the verrucae, we know that the distal ends of the animal’s thoracic legs, and the abdominal and anal prolegs, also produce painful stings if these structures are in contact with the skin when puss caterpillars sting. In the photograph below, we can see the individual thoracic legs of the anterior portion of the caterpillar’s body. This caterpillar–obtained during my study of the natural predators of the puss caterpillar–has been parasitized by a fly in the Tachinidae family; the parasitoid maggot(s) inside the caterpillar (the head of one maggot is visible beneath the caterpillar’s skin as a pale area with a dark spot–the maggot’s eye–at the center of the image) induce systemic changes that distend the body and cause the six thoracic legs, which are usually retracted, to project outward where they can be examined with ease.
Infected Puss Caterpillar legs and prolegs
Each thoracic leg (one of the caterpillar’s 6 thoracic legs is shown below in the micrograph) terminates in a conical structure that is capped at its distal end with a dark, chitinous claw-like structure, and is fitted with dark spines along and at the base of its shaft. We can only speculate on the function of these spines, as dissection of the prolegs has not yet been done. However, it seems likely that they, including the distal claw, are envenomating structures that serve to inject venom into a victim’s skin on command from the caterpillar. I use the word “command” advisedly, as while handling these caterpillars (not in direct contact with the skin, as Nathan Chandler Foot did in 1921, but with forceps) it was clear that, when irritated or threatened, they take conscious steps to attack the irritator or threat. Those steps include aggressively digging in with their claws. One is led to presume that, if the purpose of “digging in” is merely to assist ambulation, no venom would be released. However, under threat or irritation, I presume the caterpillar releases venom into the spines and claws as a means of shocking a predator into ceasing its attack.
Infected Puss Caterpillar thoracic leg details
The thoracic legs, numbering only six, provide a limited means of defending the caterpillar against predators. The abdominal and anal prolegs, however, number at least ten, and possess a much more complicated envenomation apparatus. A micrograph of the abdominal and anal prolegs of another puss caterpillar (also parasitized by a Tachinid fly) from the October-November 2009 study is shown below. As this photo shows, the distal structures of these legs present a more involved biological architecture than those of the thoracic legs shown above.
Anyone who uses forceps to pick up a puss caterpillar will likely marvel, as I have, at the tenacious way they attack, then attach themselves to, the forceps. They stick to anything, even edged, metallic instruments, like glue. Examined with the naked eye, the prolegs appear to be soft appendages unsuitable for attaching themselves with competence to anything. Under the microscope that supposition is quickly laid to rest. Each proleg is fitted with rows of minute, pointed hooks that number–for each proleg–in excess of 48 hooks. The hooks are positioned on the distal edge of a flexible, fin-like structure, and are connected to dark, bulbous swellings at the base of each fin.
Puss Caterpillar abdominal and anal proleg detail
Puss Caterpillar abdominal prolegs loseup
The obvious conclusion is that each hook is an envenomating structure and the bulbs they are connected to are venom vesicles. That is presumptuous, of course, as I have not dissected these appendages, and have not examined them under sufficient magnification to determine if they have the requisite envenomation orifices and ducts. At best, they remain the most likely candidates for the envenomations observed in humans who have had the ventral bodies of the caterpillars pressed against their skins.
Puss Caterpillar abdominal proleg Micrograph
It is appropriate to note, in connection with this, that the envenomation pattern noted in such cases, carries what appears to be the precise outline of each leg, as would be expected if these hooks are involved directly.
Links: (1) Puss Caterpillar General Information. (2) The Puss Caterpillar’s Stinging Apparatus. (3) Puss Caterpillar Extermination. (4) The Puss Caterpillar’s Natural Predators. (5). Puss Caterpillar Stings–Medical Interventions. (6) Puss Caterpillar Stings–Home Remedy First Aid Measures.
—————————————–
For additional information about the material presented on this web page, please feel free to contact Jerry via telephone or E-mail. Please also leave a detailed comment in the space provided below.