136 THE EARLY STAGES OF TABANID^ 



horizontal layers, if observed horizontally. The eggs are, moreover, 

 glued together by a substance which hardens after deposition, and 

 causes the eggs to adhere strongly to each other. It goes without 

 saying that the number of eggs contained in a horizontal layer dimin- 

 ishes if we proceed from the base of the mass towards its summit. 



The color of the egg mass, which is white at the moment of deposi- 

 tion, is changed rapidly into brown and later into black. The change 

 of coloration begins some time after oviposition is completed; it ap- 

 pears at first at the summit of the mass, thence it spreads towards its 

 base, which after a few hours has turned almost completely dark 

 brown or black. This change of coloration in the egg masses of taba- 

 nids has already been observed, as Lecaillon emphasizes, by Mann 

 and also by Hart. However, Mann, who observed the same species 

 as that studied by Lecaillon, speaks of a wax-yellow color which later 

 passes into blackish brown, and Lecaillon assumes that Mann has not 

 noticed the primitive white color of the eggs when first laid, or that he 

 possibly dealt with a different species. Hart's statement that in the 

 eggs of Chrysops moerens {cestuans) the first color is cream, seems bet- 

 ter to agree with Lecaillon' s observation. This may possibly be 

 merely an inexactness of terms, as one author may call an object 

 white which another would call pale yellow or cream. 



The change of coloration which takes place in the egg mass a 

 short time after oviposition must be considered as advantageous, as 

 we have to deal with the substitution of a protective color (black) 

 for a white color which as actually observed, renders the eggs very 

 conspicuous to the eye. 



According to Lecaillon, the color of insect eggs may be due either 

 to a coloration of the yolk, or to a coloration of the egg envelopes. 

 Changes of coloration can consequently be due to changes taking place 

 inside the egg, or to changes taking place in the envelopes. In the 

 case of Tahanus quatuornotatus , the substitution of the white color 

 by a dark color is due to a brown pigment which develops in the 

 chorion after oviposition, probably by the slow effect of air and 

 light. This pigment is very strongly developed and hides com- 

 pletely the contents of the egg. Consequently even at the end of 

 embryonic development when the eggs contained wholly white 

 larvae, they still are in external appearance completely black. 



