8o Parasitic Arthropods 



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, ase 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 

 hypo^harynx, two mandibles, two maxilla and a lower lip, or labium^ 

 made up of the fused pair of second maxillae. 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 alter 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 

 Diptera and the Siphonaptera. 



SIPHUNCULATA 



The order Siphunculata was established by Meinert to include the 

 true sucking lice. These are 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 



