Later Stages 133 



to the larval head, and form comparatively simple paired 

 folds of the dorsal epidermis (fig. 97). Behind and on 

 the ventral side is a short extension, which will subse- 

 quently give rise to the compound eye and the antennary 

 bulb. K's, the invaginations do not as yet extend into the 

 thorax, the transverse fold described above is wholly 

 wanting. In this early condition the invaginations of 

 Chironomus are essentially similar to those of Corethra 

 at the time of their fullest development. 



The prolongation of the cephalic invaginations into Later 

 the thorax gradually advances as the larva is nearing ^ ^^^^' 

 the time of pupation. The formation of the transverse 

 fold already described is a necessary consequence. This 

 fold may be regarded as an exaggeration of the slight fold 

 which in so many insects forms in the new cuticle and 

 epidermis at the junction of the head and thorax, as well 

 as between other segments of the body shortly before 

 a moult. While the backward extension of the invagina- 

 tions is taking place considerable histological differen- 

 tiation is in progress, and some change takes place in 

 the form of the future sense-organs. The compound eye 

 forms at first a vertical layer, not far from flat, occupying 

 the outer wall of the invagination. Later on, the facets 

 extend round the much bent floor of the cavity, and reach 

 to a certain height upon the inner wall (fig. gg). The 

 antenna also undergoes, especially in the male, a con- 

 siderable change of form. At first the bulb is posterior, 

 and the shaft takes a nearly straight course to the 

 larval antenna, within which its ti]3 is included ; subse- 

 quently the bulb becomes internal, and the shaft is arched 

 upwards in a bend of gradually increasing sharpness 



(fig- 95)- 



The parts of the head, thus formed within the larva, Eversim 

 assume their final position by a process of eversion 

 (turning inside out) which can be observed when the 



