of a Species of Chironomus. 35 



the head, on the pronotum, there are two thick filaments, 

 which are described by Weismann, in the pupa of Corethra 

 plumicornis^ as stigmatic branchiae {sb). Of stigmata our pupa, 

 as living in water, is entirely destitute. The tracheal system 

 is comparatively very small ; it consists of a main tube run- 

 ning through each side of the whole body, which gives off a 

 few small ramified branches, of which the thoracic branch is 

 the most considerable. The central nervous system consists 

 of eleven ganglia imited to one another by double commissures^ 

 The greater part of the abdomen is now occupied by the ova, 

 which are already perfectly developed and lie irregularly in 

 the body-cavity. But when the pupa is dissected, it is still 

 not difficult to detect the ovarian parts with the undeveloped 

 ova. 



This pupa, which has emerged from the larva in the cocoon, 

 quits the place of its birth, and after swimming about for a 

 short time, exti-udes the ova contained in it through the above- 

 described apertures situated in the penultimate ventral seg- 

 ment. These are now placed in a very regular row * (PI. III. 

 fig. 1). They are, as has been stated, imbedded in a hyaline 

 mass (a), and are attached by their whole surface to the glasses 

 of the aquarium in the form of two cords, each 2*5 millims. in 

 length. The pupa then dies ; but in some cases, after depo- 

 siting a small number of ova, it became further developed into 

 the imago. 



The case is quite different with the same pupas in the au- 

 tumn. Now, after undergoing the same development as in 

 spring, differing in nothing except perhaps the much more 

 considerable number of ova contained in them, they become 

 transformed into the perfectly developed insect, a fly of the 

 genus Chwonomus, without depositing their ova. The 

 abdomen of the escaped yellowish-green fly is shorter than 

 that of the pupa, for which reason the two hindermost abdo- 

 minal segments of the latter appear empty before the emer- 

 gence of the fly. The pupa-case then bursts on the dorsal 

 side ; the fly first of all protrudes its abdomen, then draws the 

 feet out of their tubes, then the head, and, lastly, the wings, 

 which it finally unfolds, and then flies away, probably to de- 

 posit its fertilized ova, after copulation, again in the water. 



* It is worthy of remark that, if the OTa are taken earlier out of the 

 parent organism and placed in water, they always take the same position ; 

 that is to say, they place themselves in the same order in which we find 

 them in the homogeneous mass after they have been deposited by the 

 pupa. Even when they have been scattered too far apart, a small move- 

 ment of the water suffices to cause them to arrange themselves in a row, 

 as if they possessed some attractive power. 



3* 



