198 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
floor of the buccal cavity, the hypopharyngeal sclerite. This 
structure is not homologous with the hypopharynx of the imago. 
The epithelium of the cardia and stomach is very unlike that of 
the same regions with the perfect insect, being composed of large 
polygonal cells which have a striated border and large nuclei. 
Eight cylindrical caeca protrude in a circle just behind the cardia 
and there is no valve at the posterior end of the stomach nor at the 
cephalic end of the ileum. The muscle-coats are weak. 
Of the three regions of the hind gut, the ileum has an extremely 
thin epithelium and well developed circularis muscles, the colon an 
epithelium of huge polygonal cells and slender belts of circular 
muscles at intervals, while the rectum has a very thin epithelium 
indistinguishable from that of the colon and similar belts of circu- 
lar muscles. The hind gut is lined by a strong chitinous intima. 
The metamorphosis of the mosquitoes as shown by Culex is of 
low grade compared to that of other flies. No phagocytosis has 
been observed. The fat body, the Malpighian tubules, the ventral 
thoracic muscles, and the muscles of the abdomen seemingly are 
passed on intact to the imago. The eyes of the larva are the eyes 
of the pupa and are elaborated to form part of the compound eyes of 
the adult, the remainder of these organs being developed from 
the surrounding hypodermis. The larval ocelli degenerate. The 
nervous system increases greatly in bulk and complexity, two of 
the abdominal series of ganglia change their position, a Johnston’s 
organ is formed within either antenna, and two vestigial ocelli 
appear on the vertex of the head. These changes are rather of the 
nature of growth than metamorphosis. The dermis of the head 
and body moulds itself to the pupal and imaginal contours and 
secretes the successive pupal and imaginal cuticular structures with¬ 
out visible mitosis or histolysis. The epithelium of the fore gut 
alters greatly in appearance, the pharyngeal and antlial pumps are 
differentiated, the larval esophageal valve fades away and a new 
valve is formed by the out-folding of the gut walls, and the esopha¬ 
geal diverticula are pushed out. Apparently the epipharynx alone 
undergoes histolysis during these changes. 
No true imaginal buds are developed for the mandibles, the max¬ 
illae, or the tail fans of the pupa. The hypodermis merely thickens 
and stains more deeply. The labrum develops as an evaginate 
pouch of altered hypodermis on top of the head. The labium, 
