HOW INSECTS BREATHE. 145 



greater variations : this appears to be in accordance with 

 the general law that the very young states of animals often 

 resemble in many particulars fully developed specimens 

 which belong to inferior groups. Thus, as the Crustacea 

 and many aquatic annulids respire by means of branchiae, 

 we should expect to find these organs in many larvae, and 

 in fewer pupae or nymphs. 



And so it is : many larvae, for instance, those of the 

 Gyrinidce and of Parapoynx Stratiotalis, which possess 

 branchiae, and others, such as those of the JDyticidce and 

 Hydrophilidce, are aquatic, turn into nymphs or pupae which 

 live in air. 



This rule, however, is not without exceptions : the larvae 

 of some gnats, though living in water, come to the surface, 

 and inspire air, while the pupae are provided with branchiae, 

 which generally project from the thorax. I can in no way 

 account for this exception. 



There is, however, this essential difference between the 

 gills of other water animals, as of fishes, and those of insects. 

 In the former the blood or circulating fluid acts through the 

 delicate walls of the tubes in which it runs, on the air con- 

 tained in the surrounding water, while the gills of insects are 

 membranous expansions penetrated by tracheae, which imbibe 

 the air contained in the water and carry it into the body, 

 there to be acted upon in the usual manner by the blood. 

 In what manner this is effected, by what mysterious process 

 the air is thus absorbed, is one of those questions which 

 constantly occur to Naturalists, and which we are at present 

 quite unable to answer. 



If we now endeavour to ascertain why some larvae have 

 branchiae while others are compelled to ascend to the surface 

 and obtain air from thence, we shall soon remark that, as a 

 general rule, slow, inactive, or ill-defended larvae are pro- 



1857. m l 



