WERNER MARCHAND 163 



The larvae are said to be powerful swimmers having air sacs con- 

 nected with their tracheal tubes, so that they can float or sink at 

 will. 



Large numbers of egg masses are regularly destroyed in Madras 

 by a small chalcid parasite which was not identified. 



The eggs are brownish white when deposited (Patton and Cragg). 



Tahanus stygius Say. — A species of the Middle and Southern States, 

 recorded from Massachusetts, New Jersey, Illinois, Ohio, etc., the 

 life history of which is comparatively well known through the 

 observations of Hart (1895) and Hine (1906). 



On its oviposition and egg Hine is the first to give us information. 

 The species oviposits principally on the leaves of Sagittaria stand- 

 ing in shallow water, habitually placing the eggs just above the point 

 where the petiole meets the expanded part of the leaf (Plate 1, Figs. 

 3 and 4).^^ The precision with which this habit is followed becomes 

 a matter of interest. Out of hundreds of masses of eggs observed, 

 only a few were placed on other species of plants or in a different 

 position on the leaf (Plate 1, Fig. 5). The female is occupied for a 

 half hour or more in placing the several hundred eggs composing a 

 single mass, and during this time the observer can take a position 

 close by and watch the proceedings without frightening her away, 

 but species of Tahanus are more particular about the approach of 

 intruders than are various Chrysops. 



The egg mass is white when first placed but soon turns brown; 

 it is convex, and is composed of about five layers, one above the other. 

 Individual eggs are nearly the same size as those of Tahanus atratus, 

 and are similar to them in form. Hatching as observed, occurred in 

 seven days after oviposition. From a careful study of microscopic 

 sections of eggs killed as soon as laid, it was concluded that develop- 

 ment does not begin until after oviposition, consequently the time 

 given is the entire incubation period. 



When first hatched the larvae contain a considerable amount of 

 unused yolk, which furnishes them food for a time; it is therefore 

 unnecessary for them to eat anything for a few days. This is ad- 



^^ The leaf represented in Fig. 4, is a leaf of Peltandra, similar in shape to the 

 Sagittaria leaf (Plate 1, Fig. 3). 



