62 GNATS OR MOSQUITOES — CHAl'TER III 



leucocytes, so that while fulfilling the functions of both, 

 the fluid is rather of the nature of lymph than blood. 



Nervous System. — The demonstration of the nervous 

 system can only be made by dissection, as its elements are 

 too delicate to be distinguished from the overlying intestine, 

 even in the supine position, and for this purpose specimens 

 hardened in alcohol are best. Even thus it is not altogether 

 easy to follow out any length of the chain, and for those 

 who will be satisfied to examine a closely-allied larva, it is 

 recommended to dissect the larva of one of the Ephemerida, 

 in which the nerve cord can be separated with singular ease. 

 These larva3 are very common in grassy pools and small 

 running streams, and may be recognised by their long 

 caudal bristles, which give them the appearance of possess- 

 ing a long forked tail. When one has become accustomed 

 to what to expect to see by practice on these easier subjects, 

 it will be found easy enough to teaze out the nerve cord in 

 Culex larvae. In the head, however, this is scarcely practic- 

 able by the method of teazing, and sectionising must be 

 resorted to. Taking the young larva as a type, a pair of 

 ganglia can be demonstrated for each segment, and on 

 emerging from the thorax the lateral cords separate to pass 

 backwards across the oesophagus, to combine behind it in 

 the large superocsophageal ganglion or brain. From this 

 mass filaments are given off to the eyes and antennae, and 

 from it, as well as from the ganglia of the segmental chain, 

 fibres pass to the corresponding muscles and to the peri- 

 phery, sensory filaments having been actually traced into 

 the bases of the hairs. At the time of pupation the location 

 of the ganglia undergoes changes of startling rapidity. 



Mr. F. V. Theobald notes that in a few minutes prior 

 to the escape of the pupa from the larval skin the first 

 abdominal ganglia come to lie in the posterior part of the 

 thorax, and during pupal life the changes are equally 

 rapid. In four days the fore-brain increases tenfold in 

 bulk, the first abdominal ganglia fuse with the three 

 thoracic pairs, and about the same period the eighth pair 

 shift forward and fuse with the seventh ganglia, and in 

 the ? , but not in the 3 , the double mass so formed 



