FURTHER STUDIES ON HYDROMYZA CONFLUENS 

 LOEW, (DIPTERA;.* 



By Paul S. Welch 

 INTRODUCTION 



In a previous paper ('14), the writer reported the results of 

 some observations on Hydromyza confluens, an aquatic dipterous 

 insect which occurs abundantly about Douglas Lake, Northern 

 Michigan. Parts of two seasons have since been spent in the 

 same region and additional data, as well as confirmation of 

 previously recorded observations, have been secured. The new 

 material incorporated in this paper not only aids in completing 

 our knowledge of the life history of this form but also throws 

 new light on the interesting adaptations already described. 



THE EGG 



Description. — When first laid, the eggs (Figs. 1-2) are uniformly 

 white, with a very slight tint of yellow. In clear, quiet water, they are 

 usually more easily seen when submerged on the yellow water-lily 

 petiole than when the latter is lifted from the water. They are elliptical 

 in lateral view and subcylindrical (Fig. 3) in transverse section. A 

 large number of eggs, removed from the petioles and measured, had an 

 average length of 1.69 mm., the extremes being 1.54 and 1.76 mm. 

 respectively. The maximum diameter, which is in a region well towards 

 the more acute end, has an average length of 0.35 mm., the extremes 

 being 0.30 and 0.40 mm. The ends differ distinctly in shape, one being 

 bluntly pointed while the other is more rounded and is characterized by 

 a depression in the apex. A straight, longitudinal, acute carina extends 

 from end to end, occupying the mid-position in a deep, broad, longitu- 

 dinal fossa, dividing it into two similar parts. This divided fossa com- 

 prises almost one-third of the periphery and is bounded laterad by two 

 other longitudinal carinse which extend almost parallel to the median 

 carina, converging and uniting at the ends of the egg. The effect of this 

 fossa is to give the egg a flat appearance on one side. Superficially, the 

 chorion is smooth except in the region of the longitudinal fossa. The 

 mid-longitudinal carina bears on its sides numerous minute, conical 

 spines (Fig. 5). Similar minute projections occur on and near the vertex 

 of each lateral, longitudinal carina. These minute processes are pro- 

 duced by an extra development of some of the columnar exochorionic 

 units. The chorion is approximately uniform in thickness in all parts of 



*Contribution from the University of Michigan Biological Station, No. 40, 

 and the Entomological Laboratory, Kansas State Agricultural College, Xo. 20. 



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