So 
Pa rasitic A rth ropods 
examined. These figures arc 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, arc characterized by the fact that 
the adidt 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 maxillce and a lower lip, or labium, 
made up of the fused pair of second maxilla;. 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 incom¬ 
plete metamorphosis, when the young insect, as it leaves the egg, 
resembles the adult to a greater or less extent, and after under¬ 
going 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 
Diptcra and the Siphonaptera. 
Siphunculata 
The order Siphunculata was established by Mcinert to include the 
true sucking lice. These arc small wingless insects, with reduced 
mouth-parts, adapted for sucking; thorax apparently a single piece 
due to indistinct separation of its three segments; the compound eyes 
