266 MESSES. MIALL AND HAMMOND ON THE DEVELOPMENT 



As the time of pupation approaches, the thorax becomes swollen and its segments 

 lose tlieir distinctness. The wings and legs of the future fly may now be seen indis- 

 tinctly through the larval skin (fig. 2). 



The pupa (figs. 3 & 4) is distinguished from that of most other aquatic Diptera by the 

 tufts of respiratory filaments which project from the prothorax. It lies half buried in 

 the mud at the bottom of the water, with the thorax and respiratory filaments project- 

 ing ; these are swayed and bent to and fro by the alternate flexion and extension of the 

 abdomen. After two or three days the tracheal system, which was rudimentary in the 

 larva, but is now greatly enlarged and extended, becomes filled with air, secreted from 

 the water by the help of the respiratory tufts, and the pupa floats at the surface. Some 

 of the air passes through the spiracles and inflates the pupal skin. At length the skin 

 of the back splits, the fly extricates its limbs and appendages, pauses for a moment upon 

 the floating pupa-case, as if to dry its wings, and then flies away. 



The fly (figs. 5 & 6) is a common object on our window-panes, and would be called a 

 gnat by most people. It can be easily distinguished from a gnat by its habit of raising 

 the fore legs from the ground when at rest. It is entirely harmless, and the mouth- 

 parts can neither pierce nor suck. Like many other Diptera, the flies of Chironomus 

 associate in swarms, which are believed to consist entirely of males. The male fly has 

 large plumose antennae, with their dilated bases almost in contact. In the female fly the 

 antennae are smaller and simpler, and the bases are separated by an appreciable interval. 



There are many species of Chironomus, and they are often hard to distinguish. Much 

 of our work has been done upon a species which has been identified as C. dorsalis, 

 Meig. (C. venustus, Zett.). For the identification of the species we are indebted to 

 Mr. R. H. Meade, of Bradford. C. niyroviridis, which is probably the species investi- 

 gated by Weismann, is regarded by Van der Wulp as merely a variety of C. dorsalis. 

 Our species is also that mainly employed by Meinert in his researches *. 



II. The Imaginal Disks of Weismann. 



All biologists are acquainted with the remarkable discoveries of Weismann f on the 

 development of the appendages of the fly of the Muscidae. He originally maintained 

 that the new appendages arose altogether independently of the larval cuticle and its 

 generating epidermis (hypoderm), from structures which he called imaginal disks. These 

 were described as internal ganglion-like masses of cells, each mass being enclosed within 

 a special membrane. 



A few years later Weismann added % a description of a quite distinct mode of forma- 

 tion of the imaginal appendages in Diptera. In Corethra, as he then showed, the parts 

 of the imaginal head are developed in close relation to the corresponding organs of the 

 larva, and Corethra was accordingly set up by Weismann as the type of a mode of 

 transformation distinguished from that of the Muscidse " in the sharpest manner " (am 

 schdrfsten) by the absence of imaginal disks. 



* ' De Eucephale Hyggelarver,' 1886. 



t " Ueb. d. Entstehung d. vollendeten Insekts in der Larve u. Puppe," Abhandl. Senckenb. Ges. iv. 1862-3. 



J " Die Metamorphose d. Corethra plumicornis" Zeitschr. f. wiss. Zool. Bd. xvi. 1866. 



