HARDWICKE'S SCIENCE-GOSSIP. 



157 



The larva at first appears colourless and transparent, 

 end the yolk granules show through the integument 

 a pale brown in colour. 

 When the larva is first 

 hatched, the alimentary 

 canal contains a large 

 proportion of the food 

 yolk. Older larvae assume 

 a reddish-brown colour, 

 but in both young and 

 • old, the internal parts are 

 perceptible (Fig. 60). 



The first moult occurs 

 •ehortly after the libera- 

 tion of the larva ; save in 

 increased size no diffe- 

 rence is noticeable after- 

 wards. Through how 

 many moults the larva 

 passes I do not know, 

 but long before it has at- 

 tained its full size — about § of an inch — when not quite 



two anterior and two posterior. It appears to me 

 probable that these processes, as well as the four oval 



Tig. 63. — The respiratory tissue oc- 

 curs similarly in the gth, 8th, 6th, 

 5th, and 4th segments, although not 

 marked in the figure. The line 

 shows the course of the transparent 

 colourless lines. 



a quarter that length, four long processes may be ob- 

 served from the nth segment, placed ventral laterally, 



Fig. 65.— [a) extensor diagram showing the position of the muscles of the body. Those of the 

 right side only are figured ; they occur similarly on the left. {6) Diagram showing the 

 position of the muscles of the limbs, {c), (</), and [e) Diagrams illustrating the contraction of 

 the fore-limbs. 



terminal ones, subserve respiration. Such an organ is 

 united to the nth segment in the common gnat. For 

 the evidence pointing to this conclusion, refer to what is 

 written under the respiratory system ; but I may here 

 mention, that the pulsation of the heart ceases each 

 time that the two anterior processes are jerked back- 

 ward, thus apparently cutting off their communication 

 with the segment. 



The Internal Viscera. 



The Alimentary Canal— The oesophagus penetrates 

 the 1st, 2nd, and a portion of the 3rd segment. Upon 

 each side of the 1st and 2nd, lie the salivary glands. 

 In very young specimens I succeeded in tracing the 

 ducts from these glands to the back of the mouth. 

 In the 3rd segment the oesophagus expands into a 

 thick and externally rough-walled gizzard, followed, 

 posteriorly in the 4th by a large proventriculus ; 

 this tapers gradually into a long and narrow chylific 

 ventricle which extends to the 9th segment. The 

 ventricle frequently exhibits peristaltic motion. In 

 the middle of the 9th segment six tubes enter the 

 commencement of the short intestine that follows. 

 These are the Malpighian tubes. They are somewhat 

 convoluted ; three lie upon each side the 8th, 9th and 

 loth segments. The intestine is narrower and thinner- 

 walled than the ventricle; it traverses the 9th and 

 10th ; it expands in the nth, forming a rectum, which 

 ends posteriorly in the anus at the end of the 12th 

 segment (Fig. 61). 



The Circulatory System.— & well-developed heart 

 may be observed on the dorsal aspect of the nth 

 segment, above the rectum ; it is composed of a small 

 posterior chamber, the auricle, which opens anteriorly 

 by a valve into a larger and more elongated chamber, 

 the ventricle, and the ventricle in turn opens by a 

 similar valve — preventing a returning current — into 

 the long dorsal aorta which I followed forward into 



Fig. 64. — Nervous 

 system in very 

 young larva. 



