AETHEOPODA. 103 



treillei; in Mordellal (120 species) there is said to be 25,000; in 

 the largest beetle. Dynastes Hercules, they are so numerous as only 

 to be seen by the most powerful lens. Only the rays of light 

 which enter the cones in a direct line can reach the optic nerve, 

 and yet insects have the power of discerning objects at compa- 

 ratively great distances. The larvae of insects having a complete 

 metamorphosis are said to be destitute of compound eyes. Be- 

 sides the antennas there are a pair of mandibles, often aborted 

 and two pairs of maxillae, the second pair coalescing and forming 

 the labium ; but either pair may become suctorial, and of the 

 labium only the palpi may be developed. In all orders there are 

 species with the mouths either obsolete or rudimentary. 



In all insects there are never more nor less than six legs, but 

 the fore legs are sometimes rudimentary. The leg consists of five 

 parts coxa, trochanter, femur, tibia, and tarsus. The coxa is 

 articulated to the thorax, the trochanter is attached both to the 

 coxa and the femur : the tarsus is made up of from one to five 

 joints, and almost invariably of a pair of claws ; it is either naked, 

 hairy, or scaly beneath ; the joints are sometimes furnished with 

 a dilatable membranous sac. 



Of somites the normal number is thirteen, one for the head, 

 three for the thorax, and nine (or, according to some, eleven) for 

 the abdomen ; the head, however, is assumed to consist of from 

 four to six " coalesced somites," each somite being indicated by a 

 pair of appendages. 



The nervous system consists of ganglia (varying from one to 

 eleven) connected by double commissures, giving off the nerves. 

 The supraoesophageal ganglion is sometimes called the brain, 

 with the functions of which it is analogous ; the subcesophageal 

 ganglion supplies the mouth and its appendages. 



Four or more slender cylindrical vessels, terminating at the 

 commencement of the small intestine, are supposed to perform 

 the function of a liver. They are known as " Malpighian tubes," 

 and are found in most insects. The heart is a long dorsal tube, 

 largest in the abdomen, where it is included in a saccular venous 

 sinus from which the colourless blood passes into it, and, flowing 

 towards the head, returns to the sinus by certain lacunae. There 

 are no arteries or veins. The tracheae are aeriform tubes ; they 

 commence from lateral openings, known as " stigmata" or " spi- 

 racles," and ramify over every part of the body. 



Nearly all insects undergo a metamorphosis, which may be com- 

 plete or incomplete. The pupa, when quiescent, is either entirely 

 enclosed in its case, or each limb may be more or less separately 

 enclosed ; when the pupa is active it may resemble the imago 



