HYBEUNATION OF INSECTS. 429 



first sight apparently unconnected, which at every step 

 reward the votaries of Entomology, is afforded bjr 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, requi- 

 ring exactly the same temperature, are always simulta- 

 neous. Of this fact I have had a striking exemplification 

 the last spring (1816). On the 20th of February, ob- 

 serving 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, I brought home a small branch and set it 

 in a jar of water in my study, in which is a fire daily, to 

 watch their exclusion. On the 28th of March I obser- 

 ved that a numerous brood of Aphides (not A. Betulce^ 

 as the wings were without the dark bands of that species) 

 had been hatched from them, and that two or three 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 appeared, or the 

 egg of an Aphis was disclosed in the open air. To view 

 the relation of which I am speaking with due admiration, 

 you must bear in mind the extremely different periods 

 at which many trees acquire their leaves, and the conse- 

 quent difference demanded in the constitution of the eggs 

 which hybernate 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 

 those of the Aphis of the ash ; yet in the same exposure 



