414 INSECT ARCHITECTURE. 



know that, however careful we may be in inserting a 

 cork into a glass, the mercury with which it is filled 

 is not sheltered from the action of the air, which 

 weighs upon the cork ; we know that the air passes 

 through, and acts upon the mercury in the tube. 

 The air can also, in the same way, penetrate through 

 the obstruction of a gall of wood, though it have no 

 perceptible opening or crack ; but the air cannot pass 

 in this manner so readily through the skins and mem- 

 branes of animals. 



In order to see the interior of the cavity of an 

 animal-gall, Reaumur opened several, either with 

 a razor or a pair of scissors ; the operation, however, 

 cannot fail to be painful to the cow, and consequently 

 it renders it impatient under the process. The grub 

 being confined in a tolerably large fistulous ulcer, a 

 part of the cavity must necessarily be filled with pus 

 or matter. The bump is a sort of cautery, which 

 has been opened by the insect, as issues are made by 

 caustic : the grub occupies this issue, and prevents it 

 from closing. If the pus or matter which is in the ca- 

 vity, and that which is daily added to it, had no means 

 of escaping, each tumour would become a consider- 

 able abscess, in which the grub would perish : but the 

 hole of the bump, which admits the entrance of the 

 air, permits the pus or matter to escape ; that pus 

 frequently matts the hairh together which are above 

 the small holes, and this drying round the holes 

 acquires a consistency, and forms in the interior of 

 the opening a kind of ring. This matter appears to 

 be the only aliment allowed for the grub, for there is 

 no appearance that it lives, like the grubs of flesh- 

 flies, upon putrescent meat. Mandibles, indeed, simi- 

 lar to those with which other grubs break their food, 

 are altogether wanting. A beast which has thirty, 

 forty, or more of these bumps upon its back, would 

 be in a condition of great pain and suffering, terrible 



