Of all the myriads of insects and related forms, a very few are of direct use to man, some few others have forced his approbation on account of their wonderful beauty, but the great hordes of them are loathed or regarded as directly dangerous. As a matter of fact, only a very small number are in the slightest degree poisonous to man or to the higher animals. The result is that entomologists and lovers of nature, intent upon dissipating the foolish dread of insects, are sometimes inclined to go to the extreme of discrediting all statements of serious injury from the bites or stings of any species. Nevertheless, it must not be overlooked that poisonous forms do exist, and they must receive attention in a consideration of the ways in which arthropods may affect the health of man. Moreover, it must be recognized that "what is one man's meat, is another man's poison," and that in considering the possibilities of injury we must not ignore individual idiosyncrasies. Just as certain individuals may be poisoned by what, for others, are common articles of food, so some persons may be abnormally susceptible to insect poison. Thus, the poison of a bee sting may be of varying severity, but there are individuals who are made seriously sick by a single sting, regardless of the point of entry. Some individuals scarcely notice a mosquito bite, others find it very painful, and so illustrations of this difference in individuals might be multiplied. In considering the poisonous arthropods, we shall take them up by groups. The reader who is unacquainted with the systematic relationship of insects and their allies is referred to Chapter XII. No attempt will be made to make the lists under the various headings exhaustive, but typical forms will be discussed. ARANEIDA OR SPIDERS Of all the arthropods there are none which are more universally feared than are the spiders. It is commonly supposed that the majority, if not all the species are poisonous and that they are aggressive enemies of man and the higher animals, as well as of lower forms. That they really secrete a poison may be readily inferred from the effect of their bite upon insects and other small forms. Moreover, the presence of definite and well-developed poison glands can easily be shown. They occur as a pair of pouches (fig. 1) lying within the cephalothorax and connected by a delicate duct with a pore on the claw of the chelicera, or so-called "mandible" on the convex surface of the claw in such a position that it is not plugged and closed by the flesh of the victim. [Illustration: 1. Head of a spider showing poison gland (_c_) and its relation to the chelicera (_a_).]