466 ARTHROPODA. 



mandibles and maxillae, and a fifth stylet, the hypopharynx (liy) 

 can be present. Palpi, which only occur in the Diptera, belong to 

 the maxillae (p). Reduction in number of stylets to four or three, 

 or their complete absence (some flies), is brought about by fusion 

 or by degeneration. The haustellum serves as a case for the suck- 

 ing tube, which in the Ehynchota is formed by the united maxillae, 

 in the Diptera by labrum and hypopharynx. 



The proboscis, or haustellum (the so-called tongue), of the 

 Lepidoptera (fig. 489) is a long tube coiled like a watch spring 

 beneath the head. It consists of two long grooved maxillary galea 

 firmly united by their edges. The maxillary palpi are well de- 

 veloped in the moths; elsewhere they show all stages of reduction 

 to complete disappearance. Labium and labrum are reduced to 

 small triangular plates at the base of the proboscis, the labium 

 bearing a pair of hairy palpi (pi). The mandibles are represented 

 by small plates or bunches of hair. These conditions gain in in- 

 terest when we remember that in the larva the mandibles are 

 strong biting organs, while the maxillae are small hooks, and the 

 labium is better developed only in those parts connected with the 

 silk glands, a beautiful example of relations of structure to life 

 conditions. 



In contrast to the other regions, the abdomen lacks appendages in the 

 adults. Only in the lower group of Thysanura are small lobes present, 

 behind and in the same line with the thoracic feet, which may be regarded 

 as abdominal feet. Apparently, too, the appendages of the last segment, 

 the stylets and cerci, are modified limbs, but the parts (gonapophyses) 

 used in copulation and oviposition are different in character. False feet, 

 or pro-feet, occur on the abdomen of the larvae of the Lepidoptera and the 

 Tenthredinidae, but since these are fleshy unjointed processes, it is 

 doubtful whether these are true abdominal limbs, like those of other 

 Arthropoda, or are structures independently acquired. 



Besides ventral appendages the insects usually have two pairs 

 of dorsal outgrowths upon the meso- and metathorax, the wings. 

 They are lateral folds of the chitinous coat of the notum and con- 

 tain on their interior extensions of the blood sinuses and of the 

 tracheae, which are protected by thickenings of the chitin, causing 

 the network of ' veins' or < nervures ' in the wing. Both wings 

 may be elastic, flexible, and adapted for flight, or the hinder pair 

 may alone partake of this character (true wings or alae), while the 

 first pair may be thick and parchment-like wing covers, or elytra, 

 under which the true wings are concealed when at rest. When 

 only the base of the wing is thus thickened hemelytra result. 

 Between the origins of the anterior wings is frequently a chitinous. 



