340 MANUAL OF ZOOLOGY. 



(in Pteronarcys, one of the Orthoptera, and in one of the Phas- 

 midce, of the same order) branches of the tracheae are sent to 

 variously-shaped outgrowths which are termed " tracheal gills," 

 and in which the blood is oxygenated. In all, however, with 

 the exceptions above mentioned, these temporary internal or 

 external appendages fall off when maturity is attained. The 

 wings, also, whilst acting as locomotive organs, doubtless sub- 

 serve respiration, the nervures being hollow tubes filled with 

 blood and enclosing tracheae. 



Entomologists have generally recognised the following kinds of breath- 

 ing-organs in Insects : 



1. The true tracheae, in the form of branched tubes, the walls of which 

 are strengthened by a chitinous fibre. 



2. Tracheal vesicles, or dilated receptacles directly connected with the 

 proper tracheae, but having membranous walls not supported by a horny 

 spiral fibre. 



3. The modified tracheae of some adult Hemiptera and various aquatic 

 larvae, in which the lips of the stigmata are prolonged into shorter or longer 

 external tubes, by which the air is conveyed to the interior. 



4. The tracheal gills properly so called, these usually being leaf -like 

 plates attached to the sides of the abdomen, or tuft-like processes developed 

 from the mucous membrane of the rectum. 



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-cesophageal " 

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

 and antennae. The post-cesophageat 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 may often be recognised. In the 

 adults, however, of the higher groups of Insects (such as the 

 Coleoptem, Hymenoptera, Diptera, and Lepidoptera), the thoracic 

 ganglia coalesce into a single mass. 



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 sup- 

 plied 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 movably articulated 

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



