The frequency with which they occur in man is surprising. According to European statistics they are found in 50 per cent to 60 per cent or even more. Gruby found them in forty out of sixty persons examined. These figures are very commonly quoted, but reliable data for the United States seem to be lacking. Our studies indicate that it is very much less common in this country than is generally assumed.
The Demodex in man does not, as a rule, cause the slightest inconvenience to its host. It is often stated that they give rise to comedons or "black-heads" but there is no clear evidence that they are ever implicated. Certain it is that they are not the usual cause. A variety of the same, or a very closely related species of Demodex, on the dog gives rise to the very resistant and often fatal follicular mange.
Hexapoda or True Insects
The Hexapoda, or true insects, are characterized by the fact that the adult possesses three pairs of legs. The body is distinctly segmented and is divided into head, thorax, and abdomen.
The mouth-parts in a generalized form, consist of an upper lip, or labrum, which is a part of the head capsule, and a central unpaired hypopharynx, two mandibles, two maxillæ and a lower lip, or labium, made up of the fused pair of second maxillæ. These parts may be greatly modified, dependent upon whether they are used for biting, sucking, piercing and sucking, or a combination of biting and sucking.
Roughly speaking, insects may be grouped into those which undergo complete metamorphosis and those which have incomplete metamorphosis. They are said to undergo complete metamorphosis when the young form, as it leaves the egg, bears no resemblance to the adult. For example, the maggot changes to a quiescent pupa and from this emerges the winged active fly. They undergo incomplete metamorphosis, when the young insect, as it leaves the egg, resembles the adult to a greater or less extent, and after undergoing a certain number of molts becomes sexually mature.
Representatives of several orders have been reported as accidental or faculative parasites of man, but the true parasites are restricted to four orders. These are the Siphunculata; the Hemiptera, the Diptera and the Siphonaptera.