490 INYERTBBRATE MORPHOLOOT. 



as well as in certain forms belonging to other groups which 

 have lost them through parasitism [l\iallopliaga, Pulex, Melo- 

 phagus) or other causes (Worker and Soldier Termites, Neuter 

 Ants, the females of some Moths). They are, when their 

 possessors are first hatched out, saclike structures, pro- 

 cesses of the body-wall, tracheae enclosed within blood-lacunae 

 extending from the body into their cavities. Later, however, 

 the walls of the sac come into contact, the cavity being ob- 

 literated, the tracheae with the blood-lacunae in which they 

 are situated remaining enclosed within the flat plates so 

 formed and constituting the so-called veins of the wing, which 

 have in most species a characteristic and constant arrange- 

 ment. In certain forms the anterior wings become more 

 or less thickened by the deposition in them of additional 

 chitin, and may form hard plates (dytra. Fig. 239) which 

 serve as a cover and protection for the posterior wings, which 

 in such cases are alone used in flight. In the two-winged 

 Flies (Fig. 244) the posterior wings are very much reduced, 

 being represented only by two small club-shaped structures 

 termed " balancers," attached to the sides of the metathorax. 



The body is enclosed in a chitinous cuticle, usually of 

 some firmness and frequently bearing numerous hairlike pro- 

 cesses, certain of which serve as sense-organs. Glands open- 

 ing on the surface of the body also occur in connection with 

 the integument; for example, peculiar protrusible glandular 

 sacs are situated, two or four on each segment, on the ab- 

 dominal segments of the Thysanura and CoUembola close to 

 the spurlike abdominal appendages present in those forms, 

 and are in all probability homologous with the siroilar struc- 

 tures of Scolopendrella and therefore presumably represent 

 crural glands. These glands appear, however, to be wanting 

 in other insects. Many genera of Hemiptera possess glands 

 which produce a malodorous secretion, and wax-glands occur 

 in the Plant-lice (Coccidce) and Bees, the latter also possess- 

 ing poison-glands in connection with a complicated stinging- 

 apparatus, which is a modified ovipositor. 



The respiratory stigmata vary greatly in number in dif- 

 ferent groups of Insects. In the wingless Thysanura and 

 CoUembola there are usually ten stigmata on each side of the 



