THE CLASS INSECTA, AND THE ORDER DIPTERA 25 



structure, and may even differ in the sexes of the same 

 species ; their functions are tactile, olfactory, and sometimes, 

 it is supposed, auditory. 



The three pairs of mouth-appendages or "jaws" are, 

 in successive order, mandibles, \st maxilla:, and 2nd maxillce 

 or labium. Their individual structure varies very greatly 

 throughout the class in adaptation to different modes of 

 life and different kinds of food ; all that need be said, at 

 the present moment, is that both pairs of maxillas commonly 

 carry each a pair of segmented palps, but that the mandibles 

 never do ; that the 2nd pair of maxillae are either more 

 or less fused together below and behind the mouth to form 

 a lower lip or labium, or may be modified to form a sheath 

 for the other mouth-parts ; and that the mandibles, or the 

 1st maxillae, or both of these, may be vestigial or absent ; 

 indeed, in some short-lived insects that do not feed in 

 the adult state, all the mouth-parts may be reduced to 

 vestiges. 



The Thorax and its Appendages. — The thorax is the loco- 

 motor region of the body, and is mainly filled with the 

 muscles that move the legs and wings; it also contains 

 the salivary glands and some of the largest ganglia of the 

 ventral nerve-cord, and gives passage to the gullet. Its 

 three constituent segments are not always easy to distinguish 

 one from another in insects of powerful flight, and this is 

 particularly the case with their lateral pieces or pleura. 

 In some wingless insects the posterior segments of the 

 thorax are not very sharply defined from the abdomen. 

 In the large order of Hymenoptera, where the ants and 

 bees and wasps belong, the first abdominal segment is 

 more or less intimately united with the thorax. 



The appanages of the thorax are the wings. In the 

 typical adult insect there are two pairs of these, one pair 

 attached to the mesothorax, the other to the metathorax ; 

 but in the true flies {Dipterd) only the anterior (mesothoracic) 

 pair are present, and there are numerous insects of simple 

 organisation, and many insects adapted to particular condi- 

 tions of life, that have no wings at all either in one sex 

 or in both sexes. The wings are outgrowths of the integu- 

 ment, stiffened and supported by a framework of nervures 



