ANNULOSA I INSECT A. 273 



apertures called " stigmata," or " spiracles," and ramify through 

 every part of the animal. In structure the tracheae are mem- 

 branous, but their walls are strengthened by a chitinous fila- 

 ment, which is rolled up into a continuous spiral coil. In 

 the aquatic larvae of many insects, and in one adult insect, 

 branches of the tracheae are sent to temporary outgrowths 

 which are termed " tracheal gills," and in which the blood is 

 oxygenated. In all, however, except the single insect above 

 mentioned, these temporary external appendages fall off when 

 maturity is attained. The wings, also, whilst acting as loco- 

 motive organs, doubtless subserve respiration, the nervures 

 being hollow tubes enclosing tracheae. 



The nervous system in Insects, though often concentrated 

 into special masses, consists essentially of a chain of ganglia, 

 placed ventrally, and united together by a series of double 

 cords or commissures. The cephalic or " prae-oesophageal" 

 ganglia are of large size, and distribute filaments to the eyes 

 and antennae. The post-oesophageal ganglia are united to the 

 preceding by cords which form a collar round the gullet, and 

 they supply the nerves to the mouth, whilst the next three 

 ganglia furnish the nerves to the legs and wings. In larvae, 

 thirteen pairs of ganglia, one to each segment, can be recog- 

 nised. In the imago, however, of the Coleoptera, several of 

 these primitive ganglia have coalesced, so that this number is 

 considerably reduced. 



The organs of sense are the eyes and antennae. The eyes 

 in Insects are usually " compound," and are composed of a 

 number of hexagonal lenses, united together, and each supplied 

 with a separate nervous filament. Besides these, simple eyes 

 " ocelli," or " stemmata," are often present, or, in rare 

 cases, may be the sole organs of vision. In structure these 

 resemble the single elements of the compound eyes. In a few 

 cases the eyes are placed at the extremities of stalks or pedun- 

 cles, but in no case are these peduncles movable articulated 

 to the head, as is the case in the Podophthalmous Crustaceans. 

 The antennae are movable, jointed filaments, attached usually 

 close to the eyes, and varying much in shape in different 

 Insects. They doubtless discharge the functions of tactile 

 organs, but are probably the organ of other more recondite 

 senses in addition. 



The sexes in Insects are in different individuals, and most 

 are oviparous. Generally speaking, the young insect is very 

 different in external characters from the adult, and it requires 

 to pass through a series of changes, which constitute the " meta- 

 morphosis," before attaining maturity. In some Insects, how- 



S 



