50 THE LARVA OF THE BLOW -FLY. 



involution of the epibiast. This does not appear to me to be 

 the case ; it is only the vestibule which is so formed, the tracheal 

 trunks arising in the mesoblast as solid cell strings, a fact known 

 to Weismann [2, p. 76]. This mode of development is more 

 consistent with the fact that in some aquatic larvae there are 

 no spiracles, the tracheae being closed. 



The changes of form in the posterior spiracle which occur, 

 when the larva moults, are not due to a modification of the 

 spiracle, but to the formation of a new stigmatic plate on the 

 outer side of the old one, from the hypoderm cells of the 

 vestibule. The intima of the trachea separates from the 

 external cellular layer, and a considerable space appears between 

 them, which is filled with fluid. A new intima is then formed 

 externally to the old one, the latter is afterwards shed with the 

 stigmatic plate and the other cuticular structures, and the fluid 

 is either absorbed or discharged. 



The Anterior Spiracles (Fig. ii,j) are developed before the 

 second moult, and can be recognised beneath the epidermis of 

 the very young larva as papillae on the prothoracic segment. 

 In the adult larva the anterior spiracle is fanlike : it presents 

 about thirteen minute orifices. These communicate by short 

 tubes with the tubular cavity of the spiracle, which is apparently 

 entirely filled with minute granules, forming a close yellow 

 spongy mass. I am inclined to regard this spiracle as func- 

 tionally inactive, and it is difficult to understand how it could 

 be otherwise, as it is usually buried in the decomposing fluid on 

 which the animal feeds. 



Its base is surrounded by the imaginal disc from which the 

 spiracle of the nymph is developed (Fig. n, j, id}. 



5. THE CUTANEOUS MUSCLES. 



The somatic or skeletal muscles of insects are all cutaneous. 

 In the larva of the blow- fly they closely resemble those of other 

 vermiform larvae, and form a tolerably continuous sheet beneath 

 the hypodermis. Two principal sets are easily distinguished ; 

 transverse fibres which extend from the lateral line (Fig. 4, a) 

 towards the dorsal and ventral surface of the worm, some 



