MEANS OF DEFENCE OF INSECTS. 



215 



consists of a small, thickish, circular lamina of brown silk, 

 becoming as hard as gum, which exactly fits the aperture of 

 the case, and is fixed a little within the margin. It is 

 pierced all over with holes disposed in concentric circles, and 

 separated by ridges which go from the centre to the circum- 

 ference, but often not quite so regularly as the radii of a 

 circle or the spokes of a wheel. These radii are traversed 

 again by other ridges, which follow the direction of the circles 

 of holes ; so that the two kinds of ridges crossing each other 

 form compartments, in the centre of each of which is a hole. 1 



Under this head I shall call your attention to another 

 circumstance that saves from their enemies innumerable 

 insects : — I mean their coming forth for flight or for food 

 only in the night, and taking their repose in various places of 

 concealment during the day. The infinite hosts of moths 

 (Plial&na L.) — amounting in this country to more than a 

 thousand species — with few exceptions, are all night-fliers. 

 And a considerable proportion of the other orders — ex- 

 clusively of the Hymenoptera and Diptera, which are mostly 

 day-fliers — are of the same description. One of the well- 

 known whirlwigs or water-fleas, Gyrinus^Orectocheilusvittosus), 

 differs from its congeners, according to the observations of 

 M. Robert, in running along the surface of the water only at 

 night, hiding itself under stones on the banks by day. 2 

 Many larvce of moths also come out only in the night after 

 their food, lying hid all day in subterraneous or other 

 retreats. Of this kind is that of Fumea pulla and Nycte- 

 robius, whose proceedings have been before described. The 

 caterpillar of another moth {Noctua subterranea F.) never 

 ascends the stems of plants, but remains, a true Troglodyte, 

 always in its cell under ground, biting the stems at their base, 

 which falling bring thus their foliage within its reach. 3 



The habitations of insects are also usually places of retreat, 

 which secure them from many of their enemies : but I have 

 so fully enlarged upon this subject on a former occasion, that 

 it would be superfluous to do more than mention it here. 



1 Reaum. iii. 170. De Geer, ii. 519. 545. 



2 Ann Soc. Ent. de France, iv. bull. lxxx. 



3 Fab. Ent. Syst. Em. iii. 70. 200. 



p 4 



