CLASS INSECTA 



283 



the result of rubbing rough surfaces together. The locust rubs the 

 femora of the hind legs against the outer surfaces of the tegmina or 

 produces the crackling sound heard in flight by rubbing the front edges 

 of the hind wings against the tegmina. The shrilling of a male cicada is 

 due to the vibration of a tense membrane drawn across a sound chamber 

 on the first abdominal segment above the hind leg; its vibration is con- 

 trolled by special muscles. The buzzing of many insects is due to rapid 

 vibration of the wdngs. The auditory organ of a locust is a pit, or 

 tymyanum, on the first abdominal metamere, closed by a thin kidney- 

 shaped tympanic membrane (Fig. 181). Organs believed to be auditory 



•<. II 



A B c 



Fig. 188. — Nervous systems of three insects to illustrate condensation and cephaliza- 

 tion. A, a termite, one of the lowest insects. B, a water beetle. C, a fly, the highest type. 

 {From VanCleave, "Invertebrate Zoology," A after Lespes, B and C after Blanchard, by the 

 courtesy of McGraw-Hill Book Company, hic.) 



also exist on the antennae of many insects and on the legs of katydids. 

 A few insects, especially the larvae and adults of certain beetles known 

 respectively as glowworms and fireflies, are luminescent. The emission 

 of fight, fike the utterance of sounds and the production of odors, probably 

 serves to bring the sexes together. 



315. Reproduction. — Insects are always diecious. The eggs, which are 

 fertiUzed internally, undergo superficial cleavage and develop much as do 

 those of the crayfish. From the egg hatches a larva which differs in char- 

 acter in the various groups. Most of the insects exhibit a metamorphosis. 

 Only a few do not do so, and these are termed ametabolous (Fig. 189) and 

 are grouped together as Ametabola (am e tab' o la; G., ametabolos, 

 unchangeable). Most of them pass through several stages, but the num- 

 ber of stages varies. In all insects the larval form is the period during 

 which growth takes place, no insect growing after it has become adult. 



