INTRODUCTORY REMARKS. 3 



exist in the mouths of insects,* we shall have an idea of 

 the larva of the fly, commonly known as the gentle or maggot : 

 which, although it resembles one of the lowest of the articulata, 

 except in the nervous system, as will be seen afterwards possesses 

 the power of development into one of the highest of its forms. 



The integumental system of this larva consists of seventeen 

 segments,! of which the three anterior alone have lost more or 

 less of their original annular form. These, both in the larva and 

 perfect insect, compose the mouth, and in both are provided with 

 three pair of modified limbs, although the existence of the first 

 pair is less certain in the larva, owing to the very rudimentary 



- Each segment in the lowest articulata is normally furnished with 

 two pairs of lateral appendages or rudimentary limbs, one pair placed 

 above the other, the superior being dorsal and the inferior ventral ; at 

 least, such is their arrangement in annelides. Both pairs are 

 much modified in the higher forms, and are often entirely suppressed. 



The segments themselves may be said to consist typically of four plates, 

 a ventral, a dorsal, and a lateral plate on each side ; the superior appen- 

 dages being placed between the lateral and dorsal, and the inferior 

 between the lateral and ventral plates. In insects the wings, when they 

 exist, represent the dorsal appendages, being placed between the superior 

 and lateral plates. It seems to me probable that the spiracles and their 

 tracheae are likewise lateral appendages. If such be the case, I think it 

 quite likely, from the number of lateral appendages, that more than 

 three segments really exist in the thorax, especially as the anterior 

 thoracic spiracles are placed between two segments and the four posterior 

 legs occupy a similar position; so that possibly these may be latera' 

 appendages of undeveloped segments. This is certainly the case if the 

 spiracles with their tracheae are homologues of lateral appendages, like the 

 wings and legs. This appears to me not unlikely from their position^ 

 and further from the consideration that the lateral appendages of many 

 annelides are both respiratory and motor organs, ( M. Quatrefages Hist 

 Nat. des Anneles Nouv, Suites a Buffon, Tom. i. p. 19). And we must 

 remember the tracheae of insects are merely modified processes of the skin 

 developed inwards instead of outwards. 



t This is the typical number assigned on theoretical grounds to all 

 true insects, which always have the body in the perfect state divided into 

 three part*, the head, thorax and abdomen. The head consists of five 



B 2 



