434 HYBERNATION OF INSECTS. 



at first sight apparently unconnected, which at every 

 step reward the votaries of Entomology, is afforded 

 by the coincidence between the period of the hatching 

 in spring of eggs deposited before winter, and of the 

 leafing of the trees upon which they have been fixed, 

 and on whose foliage the larvae are to feed : which two 

 events, requiring exactly the same temperature, are 

 always simultaneous. Of this fact 1 have had a striking 

 exemplification the last spring (1816). On the 20th 

 of Februapy, observing the twigs of the birches in the 

 Hull Botanic Garden to be thickly set, especially about 

 the buds, with minute oval black eggs of some insect 

 with which I was unacquainted, 1 brought home a 

 ijraall branch and set it in my study, in which is a fire 

 daily, to watch their exclusion. On the 28th of March I 

 observed that a numerous brood of Aphides (not^. Be- 

 tulee, as the wings were without the dark bands of that 

 species) had been hatched from them, and that two or 

 thiee of the lower buds had expanded into leaves, upon 

 the sap of which they were greedily feasting. This 

 was full a month before either a leaf of the birch ap- 

 peared, or the egg of an Aphis was disclosed in the 

 open air. — To view the relation of which I am speak- 

 ing with due admiration, you must bear in mind the 

 extremely different periods at which many trees ac- 

 quire their leaves, and the consequent difference de- 

 manded in the constitution of the eggs which hyber- 

 nate upon dissimilar species, to ensure their exclusion, 

 though acted upon by the same temperature, earlier 

 or later, according to the early or late foliation of these 

 species. There is no visible difference between the 

 conformation of the eggs of the Aphis of the birch and 



