1876.1 257 



Xoles on Acentropus. — At a meeting of the Nctherlaud Entomological Society, 

 held at Leyden on the 18th December laat, Mr. Ritsema made the following obser- 

 vations on two points in the life-history of Acentropus niveus, Olir., namely, the 

 mode of pairing in the species in question, and the connection existing betwon the 

 two forms of female (the rudimentary and tho normally winged) and the different 

 broods. 



As regards the pairing, the female, according to an obserTation of Reutti, sub- 

 merges herself during the act of pairing, and even draws the male under water with 

 her. Although 'Sir. Ritsema had not actually witnessed the act, he considered himself 

 justified in coming to the conclusion that this takes place not in, but on, tho water. 

 On the 1st June last, in the evening, he found two pairs of A. niveus had developed 

 in his aquarium, the females with rudimentary wings. Tho aquarium was situated 

 in the garden, and the wind that evening was so high that he found it impossible to 

 keep the lantem, which he used on the occasion, alight for more than a few moments, 

 80 that ho was afraid he should not be able to witness the copulation even should it 

 take place, and, in fact, he only had time to observe the males hovering about the 

 females which were floating on the surface of tho water ; in consequence of the storm 

 increasing, Mr. Ritsema did not visit the aquarium again that night. On the following 

 morning tho males were, as usual, just above the surface of the water on the stems 

 of some plants, the females being below the surface, on the leaves of Potamogeton, in 

 close proximity to a number of eggs, which subsequently turned out to be impregnated 

 eggs of Acentropus. Mr. Ritsema supposes from this, that the act of pairing takes 

 place on the surface of the water, and that the female then dives down to lay her 

 eggs on the leaves of the food-plant. Probably tho pair observed by Reutti was, 

 somehow or another, disturbed, and the female considered it advisable to dive down 

 before the action of the male was completed. 



In order to appreciate the connection which probably exists between the two 

 forms of female and the different broods, it is necessary to pass in review a complete 

 cycle of the development of the insect. A female with rudimentary wings appears 

 at the end of May, and consequently belongs to a brood which may properly be 

 called a spring brood. Tliis female, after having paired, deposits her eggs ; and, 

 from a part of the larva) produced from these eggs, imagos are developed during the 

 same summer: these form the second or autumn brood, the females of wiiieh, 

 according to tho example raised by Mr. Ritsema, appear to be furnished with only 

 the rudiments of wings ; the remaining larvae hibernate. Tho imagos of the second 

 or autumn brood pair, the females lay their eggs, and from these larva; are produced 

 before tho winter, wliich lurvx-, consequently, liibernate at a very early stage of their 

 existence, together with a part of the larva) derived from tho spring brood. An 

 immediate consequence of the dissimilar age of the hibernating larvte is that in the 

 following year the descendants of the spring brood will develop into tho imago state 

 sooner (thus again forming the spring brood with rudimeutarily-winged females) 

 than the descendants of the autumn brood ; and Mr. Ritsema supposes that from 

 tho laat mentioned larvio a generation appears about the middle of the summer, the 

 females of which possess normally developed wings, so that he comes to tliia conclusion, 

 namely, that the normally-winged females of Acentropus must belong to that gener- 

 ation (it might be called the summer brood) which is produced from the autumn 



