INSECTA. 



983 



state, are almost or entirely imperforate in the 

 perfect. The reason for this appears to result 

 from the changes that take place as regards 

 the region of the body in which respiration 

 is chiefly carried on in the two states of 

 the insect. In the larva state respiration is 

 carried on chiefly in the abdominal region, but 

 in the perfect the chief part of the body con- 

 cerned is the thorax. It is through the tho- 

 racic and first pair of abdominal spiracles that 

 nearly the whole of the air enters and is ex- 

 pired at each act of respiration, and conse- 

 quently it is found that the spiracles in those 

 parts are very much larger than in the abdo- 

 men. The largest spiracle is usually the pro- 

 thoracic, of which we have an example in 

 Geotrupes and Gryllutalpa, and the next 

 largest, as in Geotrupes, is the first abdominal. 

 The situations in which the spiracles are placed 

 also vary considerably in different insects. In 

 Coleoptera and Orthoptera the first pair are si- 

 tuated in the membrane between the pro- and 

 meso-thorax, and the remaining ones in the 

 meso-* and meta-thorax and following seg- 

 ments of the abdomen. There is also a similar 

 arrangement of the spiracles in Hemiptera. 

 But in other insects, as in the larvae of Lepi- 

 doptera, the first spiracle is situated in the pro- 

 thorax, and the remaining eight pairs in the 

 fifth and succeeding segments to the twelfth ; 

 while in the larvae of Hymenoptera, in which 

 there are ten spiracles on each side, they are 

 placed in the second, third, fifth, sixth, and 

 succeeding segments to the twelfth. 



The second form of external respiratory or- 

 gans, as Burmeister remarks, are simply elon- 

 gated spiracles, and are found only in those 

 insects which reside almost constantly in the 

 water, but breathe pure atmospheric air, for 

 which purpose they come to the surface of the 

 water at intervals to respire. They are short 

 horny tubes, which in some instances are sur- 

 rounded by plumose setae. They are always 

 open at their extremity, and in general project 

 beyond the body. They are chiefly met with 

 in the aquatic Hemiptera, as in Nepa (Jig. 

 352) and Ranatra, and are usually two in 

 number, projecting from the extremity of the 

 abdomen. In Nepa they are about half the 

 length of the body, and in Ranatra as long as 

 the whole body itself. It is through these 

 tubes that the whole of the respiratory function 

 is performed, and the air both inspired and ex- 

 pelled. They exist also in the larvae of the 

 DyticidtE and Hydrophilidtf, in which they 

 are the only respiratory passages, although it 

 has been thought by some that lateral spiracles 

 exist also in these larvae, as subsequently found 

 in their perfect insects. This form of respira- 

 tory organ exists also in the aquatic larvae of 

 some Diptera, as in the rat-tailed maggot, 

 Eristalis, and the larva of Stratiomys. In the 

 latter instance the insect supports itself at the 

 surface of the water by a coronet of radiating 

 setae, in which it includes a bubble of air, and 

 descends with it to the bottom of pools for 

 the purposes of respiration, and comes again 



* Straus. 



to the surface for a fresh supply when the store 

 it has carried with it is exhausted. These or- 

 gans are thus distinct from those by means of 

 which the insects respire the air mechanically 

 mixed with the water. 



Bronchia constitute the third form of ex- 

 ternal respiratory organ. This form is met 

 with only in the larva and pupa state. These 

 organs are always situated at a part of the body 

 at which the spiracles are subsequently to exist. 

 They are formed, as in the larvae of Amphibiae, of 

 extensions outwards of the exterior or cuticular 

 surface of the body, and are largely supplied 

 with bloodvessels, and tracheae ramify within 

 them. They are never, as in the gills of fishes, 

 developed internally, excepting when they exist 

 at the anal extremity of the body, as in the 

 Libellulidte, but are extended from the sides as 

 in the Tadpole, being simply expansions of the 

 external surface. \Ve are not aware that cilia 

 have yet been observed on these surfaces, but 

 judging from the analogies of structure and 

 formation that exist between these parts in 

 insects and the analogous ones in the larvae of 

 Amphibia, there seems reason to expect that 

 they do probably exist. The necessity for such 

 structures on the branchiae may, however, be 

 rendered less imperative from the voluntary 

 power which the insect itself possesses of 

 moving the branchiae at pleasure, by which 

 the function of cilia, that of effecting a con- 

 stant renewal of the water in contact with the 

 surface of the organ, is steadily accomplished. 

 It is the belief of most entomologists,* that 

 branchiae absorb the air from the water, and 

 convey it by the minute ramifications of the 

 tracheal vessels, with which they are abundantly 

 supplied, and which terminate in single, trunks, 

 into the main tracheae, to be distributed over 

 the whole body, as in insects which live in the 

 open atmosphere. This is supported by the 

 fact that the tracheal vessels, as seen in the 

 transparent bodies of many aquatic larvae, are 

 filled with air; but the subject still admits of 

 enquiry, why in these instances the usual func- 

 tion of branchiae is so far departed from as to 

 allow of the air being absorbed from the water 

 into distinct vessels, to be distributed over the 

 whole body, for the purpose of aerating the 

 fluids, rather than that it should be brought 

 into contact with the blood, and undergo the 

 consequent changes at the surface of the bran- 

 chiae, as in the larvae of Amphibia and Fishes. 



The branchiae of insects are of three kinds. 

 The first exists in the form of elongated, 

 slender, hair-like organs, collected together in 

 tufts, that originate by single stems, as in the 

 larvae and pupae of gnats. f This form is by far 

 the most common. These filamentous parts 

 are supplied each with a single trachea that is 

 extended throughout their whole length, and 

 which is connected with the great longitudinal 

 tracheae of the body. In a very few instances, 

 branchiae of this form originate separately, and 

 not in tufts, as, according to De Geer,| in the 



* Burmeister. 



t Burmeister, op. cit. p. 167. 

 t Memoires sur les Insectes. vol. iv. pi. 13, 

 fig. 16.19. 



