160 Journal New York Entomological Society, f^oi. xxv. 



is, when the embryo is exactly four days old, the final body segments 

 begin to be visible chiefly in the anterior part of the body, and the 

 head and pharynx is seen to be in the retracted condition, while the 

 first body segment reaches to the anterior end of the tgg. Pharynx 

 and intestine have become more distinct; the yolk mass has shrunk 

 further, occupying now less than one half of the whole length, and 

 not reaching the end of the body. The trachese and the organ of 

 Graber are not yet visible in this individual (Plate XII, fig. 7). How- 

 ever, in a second individual from the same egg-cluster, the organ of 

 Graber is seen already fully developed and also the tracheae are dis- 

 cernible (Plate XII, fig. 8). The head is seen retracted as in the 

 first-studied individual. Concerning the organ of Graber, it should 

 be noted that if this organ really corresponds to a pair of modified 

 hairs, as assumed by G. Paoli, it should always be simple in the 

 embryo, as it is in fact in Chrysops. However, it seems that in the 

 young Tabaiius conditions are somewhat different. The body seg- 

 ments in the larva (Plate XII, fig. 8) are seen fully marked; the hind 

 end of the body is slightly longer than the space afforded to it, and 

 the last segment is curved in order to find room in the egg. All 

 through these stages we notice a gradual increase in length of the 

 whole embryo in the egg-shell, with the result that even with head 

 retracted within the first segments, and with tail curved backward, 

 it fills the whole available space. This may be of considerable im- 

 portance in the act of hatching from the egg, as it enables the 

 embryo to burst the egg-shell by simply stretching its body. On 

 Plate XII, figs. II and 12, these movements are illustrated. 



On the following day, July 12, 4 P. M., the eggs had not yet 

 hatched. However, if the eggs are placed on a slide, a slight pressure 

 of the coverglass is sufficient to cause them to leave the egg with 

 vigorous movements. The young larva (Plate XII, fig. 9) is seen to 

 be a typical Chrysops larva, eleven-segmented, with seven pairs of 

 lateral prolegs which are not yet protruded, with two slender tracheal 

 trunks forming loops in the anterior part of the body. The head is 

 retracted and remains so in the larvae which were caused to hatch 

 prematurely. The yolk mass is still present but takes less than one 

 third of the whole body length. The lower part of the intestine shows 

 irregular windings similar to those seen in C. caUidns. The organ of 

 Graber is distinctly seen immediately behind the dorsal bloodvessel, 



