OF THE HEAD OF THE IMAGO OF CHIRONOMUS. 273 



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 invaginations is taking place 

 considerable histological differentiation 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 (fig. 19). Later on, the 

 facets extend round the much bent floor of the cavity, and reach to a certain height 

 upon the inner wall (PI. XXX. figs. 21, 25, & 28). This change appears to be due to 

 a modification of the shape of the invagination rather than to a development of new 

 facets, for in late stages there is a marked change of shape in transverse sections of 

 the invaginations (compare PL XXIX. figs. 18, 19, & 20, with PI. XXX. fig. 28, and 

 PI. XXXI. figs. 31 & 32). The antenna also undergoes, especially in the male, a consi- 

 derable change of form. At first the bulb is posterior (PL XXIX. fig. 22) and the shaft 

 takes a nearly straight course to the larval antenna, within which its tip is included ; 

 subsequently the bulb becomes internal, and the shaft is arched upwards in a bend of 

 gradually increasing sharpness (PL XXX. fig. 25). 



VI. The Process of Pupation in Chironomus. 



Larvae about to undergo pupation can be easily distinguished by the thickened thorax. 

 If a number of such larvae are observed continuously for a few hours, the process of 

 pupation can be studied without serious difficulty. The first distinct sign of change is 

 the retraction of the epidermis and soft parts from the old cuticle of the prothoracie 

 feet. Very shortly after this (about a minute) the same process takes place in the anal 

 papillae, the respiratory tubules, and (a little later) in the anal feet. After a further 

 interval of a few seconds, or at most a minute or two, the head and prothorax of the 

 pupa protrude from the dorsal surface, between the larval head and prothorax. The 

 larval head, which has been suddenly emptied by the retraction of its contents, then 

 slips round to the ventral surface. The exact order of these events is not quite constant. 

 Now and then the anal feet and other posterior appendages are seen to be unchanged in 

 a larva which has already slipped off the larval head, but this is unusual. It is probable 

 that the contraction of the prothoracie and anal regions sets up a blood-pressure, which 

 is the immediate agent in the protrusion of the head. An independent indication of the 

 existence of such blood-pressure at the time of pupation is given by the occasional 

 escape of a large quantity of blood, which fills the space between the old cuticle and the 

 retracted epidermis. In such cases we have found that the pupa dies within a short 

 time. The complete removal of the larval cuticle from the body is a matter of time, and 

 may occupy several hours. The old cuticle becomes much wrinkled, and is ultimately 

 torn into shreds, being gradually rubbed off by the almost incessant movements of the 

 pupa. 



Sections taken through the pupal head a little after the time of change illustrate the 

 eversion of the imaginal head. The compound eyes, which were deeply invaginated, 



SECOND SERIES. — ZOOLOGY, VOL. V. 41 



