1) Tempting as it may be, picking up nice fuzzy caterpillars may not be a good idea. The body surface of some caterpillars, and a few adult insects, can be covered with urticating hairs. These urticating hairs are used as a defense mechanism against predation.
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2) Urticating hairs can be broken down into two categories: Envenomating (venomous) and non-envenomating (irritating) hairs.
3) Envenomating hairs usually take on the form of sharp hollow bristles seated ontop of a fluid secreting gland. These hairs break under pressure, penetrate into the skin and release their fluid. The fluid (usually a mixture of histamines) can cause irritation or inflammation. (source: http://medent.usyd.edu.au)
4) Non-envenomating hairs are easily dislodged when disturbed and can become airborne, settling on nearby surfaces. They are designed to produce a mechanical irritation that is often described as having similar effects of fibre glass particles on the skin. Coming into contact with these hairs usually results in a skin irritation. (source: http://medent.usyd.edu.au)
5) Urticating hairs can take many forms, they can be bristle-like seate or sharp-pointed hollow bristles, while others can take on the form of darts, long flexible tapering hairs, or rigid bristles.
6) The urticating hairs can be arranged in a distinct pattern or tubercle, or they may cover the insect in its enterity. The location and structure of the hairs can be used as an identifcation tool.
Have you ever had an allergic reaction to caterpillars? Ever handled a caterpillar and not have a reaction?