54 



AN INTRODUCTION TO ENTOMOLOGY. 



Owing to the small size of these insects, the majority of them 

 escape the attention of all except the more careful students of 

 nature. The order is, however, of great interest to entomologists ; 

 for it includes the lowest or simplest of the true insects ; and in it 

 are found forms which show close affinities to the next lower class, 

 the Myriapoda. It is thus one of the connecting links of which we 

 hear so much in these days; and in it are, doubtless, forms which 

 more closely resemble than any other living species those which in 

 ancient geological times were the first Hexapoda to appear on the 

 earth. 



The low rank, of these insects is indicated in many ways. The 

 mouth-parts are of a primitive form ; wings are never developed ; 

 and the insects undergo no metamorphosis, the larval form being 

 retained by the adult. 



The absence of wings in this order is believed to represent the primitive con- 

 dition of these insects. None of the species show any indication of the devel- 

 opment of these organs. And the thorax does not present that complication 

 of structure which is the result of the development of wing-muscles. In each 

 of the higher orders we find wingless species; but in these cases there is good 

 reason for believing that the wingless condition is the result of a retrograde 

 development. In some cases this degradation is the result of parasitic habits, 

 as with lice, fleas, and mady other parasites; in other instances it is the result 

 of the separation of the species into several castes, of which some do not re- 

 quire wings, as the workers and soldiers among Termes, and the sedentary 

 generations of the Aphides. 



Upon the distinction given above Professor Brauer separates the insects into 

 two classes. The first includes only the Thysanura ; this he calls the Aptery- 



gogenea, or " originally wingless insects ;" 

 it coincides with the super-order Syndp- 

 tera of Packard. The second class Brauer 

 terms the Pterygogenea, or " originally 

 winged insects." 



The form of the mouth-parts of 

 the Thysanura is very different from 

 that seen elsewhere in the class 

 Hexapoda. Here the mandibles 

 and maxillae, although fitted for bit- 

 ing, are retracted within the head, 

 instead of being attached externally 

 as is the case with the higher in- 

 bial palpus - sects whose mouth-parts are formed 



for biting. Fig. 58, I, shows the relation of the jaws to the wall of 

 the head in a common spring-tail, one of the Entomobryidce. In this 



FIG. 58. Mouth-parts of a Spriner-tail, Ento- 

 (Drawn by J. M. Stedman, un- 



