188 B. T. LOWNE ON THE HYMENOl'TEKA. 



All this is very remarkable, but the forms which I am going to 

 describe are more remarkable still, for they are not only parasitic, 

 but they prey upon one another. In size they are scarcely visible 

 to the naked eye, they would in fact only look like grains of small 

 dust. I am going to bring under your notice some facts and 

 observations of extreme interest in relation to the development of 

 three genera, all the species of which are parasitic, and all of which 

 have fringed wings, characteristics, however, I believe, common to 

 a vast number of genera in this class of Hymenoptera. 



The first genus which I shall describe is known as Platygaster, 

 and in tracing up its development I shall begin with the egg. The 

 eggs of most of these curious little Hymenoptera are enclosed in an 

 envelope which has a small pedicle attached to it ; they are of 

 com'se exceedingly minute, are transparent, and contain no food 

 yolk. In the bird's egg, it will be remembered, there is a large 

 yolk, and there is also a portion known as the tread or cicatricula. 

 This is the true germinal spot of the egg ; it is living matter, and 

 the food yolk feeds this little piece of protoplasm during the pro- 

 cess of development. Moreover, in the case of a large number of 

 worms the egg is formed of a union of two kinds of yolks secreted 

 by different glands and only united in the egg ; but the eggs of 

 these flies contain no food yolk, only a protoplasmic yolk, and the 

 reason for this is explained by their curious mode of development. 



In the course of development, we first find that the single cell 

 is broken up into three cells, and a very curious thing is that the 

 central cell begins to form young cells inside it, and that the other 

 cells form an envelope round the central cell. We may look upon 

 this as the formation of the first shed skin of the creature. Whilst 

 this is taking place, large cells are being formed in the interior of 

 the central cell ; from these the larva of platygaster is developed. 



The eggs of Platygaster are laid in the body of the larva of one 

 of the diptera, Cecidomyia, in which it undergoes the process of 

 development. It is at first a solid mass of protoplasm, as there is 

 no central fluid food yolk, but after the above-described changes, 

 the central mass of cells becomes inflected precisely in the same 

 manner as the embryo becomes folded in the course of the develop- 

 ment of the egg of the lobster or crawfish, and, as in the case of 

 the crawfish, one side of the depression forms the head and the 

 other the tail. This change goes on until at last an embryo is 

 formed, bearing a strong resemblance to a rudimentary crustacean, 



