76 ON PSYCHOPTERA PALUDOSA. 



size, but also in the absence of the toothed crown, its place being 

 taken by a simple infolding of the external wall to form the 

 extremity of the trachea, a constriction occurring at this point to 

 prevent the access of water, see Fig. 1 7. It has been asserted by 

 Mr. Lowne* that the spiracles of insects, together with the tracheae, 

 are invaginated lateral appendages, similar to the wings and legs, 

 developed inwardly instead of outwardly. If we regard the pro- 

 bable course of development of the functions of the body in the 

 animal kingdom we may conclude that, as the function of respira- 

 tion probably preceded even that of locomotion, as being the more 

 necessary to the existence of a living being, the converse of this is 

 rather the case, and that the appendages, at least the superior ones — 

 namely, the wings, halteres, and the pro-thoracic pupal appendages 

 in question — are externally developed respiratory organs, in many 

 cases diverted from their original function ;t but be this as it may, 

 there appears to be a striking amount of similarity and of con- 

 tinuity in the coils of the external wall of the filament of Fsychop- 

 tera to the finer spirals of the enclosed trachea, which suggests 

 that the whole organ is a trachea pushed outwards. 



I must conclude with Lyonet's account of the perfect insect> 

 see Fig. 4. He says,: " Its predominant colour is black ; the legs 

 and extremity of the abdomen are the colour of dead leaves ; the 

 antennae are black, and composed of sixteen knots, furnished with 

 very minute hairs. Every joint of the legs is marked with a black 

 spot. Its most notable peculiarities are, however, first, the form of 

 its body, the base of the abdomen being very slender, and this is 

 followed by three swollen rings, the last of which ends in a 

 point; and secondly, the painting of the wings, the nervures of 

 which are not only very black, but are adorned with numerous 

 spots of the same colour, giving it a very ornate appearance." 



* II 



Anatomy of the Blow Fly," p. 3, note. 



t In this way we should view the incipient wings of the Ephemera larva as 

 modified tracheal gills, and not both these organs as differentiations from originally 

 indifferent appendages. 



