INSECTA 



191 



three straight fissures, in Musca three crooked ones. These fissures 

 are the orifices leading to the tracheae. The posterior field upon 

 which these stigmata are placed is convex in Musca, slightly concave 

 in Calliphora, and very much sunken in Sarcophaga. In the latter 

 species, its thickened edge (fig. 100, a) bears numerous pointed, fleshy 

 growths which are easily seen with the glass. These growths are absent 

 in Musca. Ventrally of them in Sarcophaga is the anal lamella, which 

 varies in shape in different species. 



The stigmatic structures do not remain un- 

 changed during the entire course of larval life ; 

 on the contrary, they undergo three develop- 

 mental changes. In the first stage, when the 

 larva emerges from the egg, both longitudinal 

 tracheal tubes are present, and have their 

 orifices placed posteriorly, as in the older larvae. 

 The stigma is simpler, however, than in the older 

 larvae. In Musca, it appears as a hole with a 

 heart-shaped border ; while in Calliphora and 

 Sarcophaga, it is a true stigmatic lamella, but 

 furnished with two, not three, fissures. If the 

 external temperature is sufficiently high these 

 larvae will slough their skin at the end of twenty- 

 four hours, passing into their second develop- 

 mental stage. Two anterior stigmata, of some- 

 what different structure, now appear upon either 

 side of the second segment. The longitudinal 

 canals of the tracheal system divide at the 

 body-wall into a fan-shaped arrangement of short branches, which 

 project more or less beyond the body and vary in number and 

 shape according to species. After the first sloughing, the posterior 

 stigmatic apparatus becomes larger, with more clearly defined border 

 and, in the species here described, is generally furnished with two 

 straight fissures. It is not until after the second sloughing that the 

 posterior stigmatic apparatus assumes its final form, that, namely, 

 of three somewhat convergent fissures which, in Calliphora and 

 Sarcophaga, are straight or very slightly curved, and in Musca are 

 serpentine. 



It should be noted in passing that, when the larva sloughs for 

 the third time, the skin is not thrown off, but is retained and, after 

 becoming shorter and very much browner, forms a resistant envelope 

 in which the pupa is enclosed. 



Certain changes also take place in the mouth-parts, especially in 

 those of the larvae of Musca. These, as well as certain peculiarities 

 of the chitinized body parts, are best seen in larvae which have been 

 treated with potassium solution. The action of the fluid will be 



FIG. 99. Larvae of 

 Calliphora vomitoria. 

 Magnified. 



