AMERICAN MOSEOUM GOIDE LEAPLE AS. 

CHARACTERISTICS OF INSECTS 
First of all, a word should be said as to the characteristics of these 
creatures and their place in Nature. Those animals which have no 
internal skeleton but do have, at some period of their lives, jointed 
legs are called Arthropods. Familiar examples are lobsters, spiders, 
centipedes, and insects. We are now chiefly concerned with certain 
insects but we must also consider mites and ticks, creatures which are 
more closely related to the spiders. An insect has its body divided 
into three regions: head, thorax, and abdomen. Its jointed legs are 
borne by the thorax, the segments of the body which are just back of, 
but separated from, the head; there are never more than three pairs 
of such legs in an adult insect. Spiders, mites, and ticks have, typi- 
cally, four pairs of jointed legs, and the head is merged with that part 
of the body which bears the legs. 
The great majority of insects are winged, when adult, and most 
winged insects have two pairs of wings, but the members of the large 
Order Diptera, to which mosquitoes and flies belong, have never 
more than one pair. Nearly all of the strictly parasitic insects are 
wingless, even when adult. We will take up first those disease-bearing 
insects which have, when adult, one pair of wings, next certain 
wingless insects, and finally the mites and ticks. 
Two sorts of Diptera or two-winged insects are of interest in the 
present connection: (1) ordinary “‘flies’’ with three-jointed antenne; 
and (2) mosquitoes, gnats, etc., which have eight or more freely 
moving joints in each antenna. The Muscide and the Tabanide 
(p. 13) are of the first sort. 
The Muscide, a group which includes our commonest disease- 
bearing insect, the filth fly, are characterized as follows: 
The squame (see Fig. 1) of Diptera are scale-like structures placed 
back of the roots of the wings and above the knobbed “‘balancers’’; 
and the Muscide agree with the related Diptera in having these 
squame large. The auxiliary vein in the wing (see Fig. 1) is distinct 
in its whole course, and the first longitudinal vein is never very 
short. The thorax has a complete transverse suture. The eyes of 
the male are usually much nearer together than those of the female; 
sometimes, in fact, so close that they touch each other. 
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