MEANS OF DEFENCE OF INSECTS. 463 



certain saw-flies spin for themselves a cocoon of a soft, flexible, and close 

 texture, which they surround with an exterior one composed of a strong 

 kind of net-work, which withstands pressure like a racket.^ Here nature 

 has provided that the inclosed animal shall be protected by the interior 

 cocoon from the injury it might be exposed to from the harshness of the 

 exterior, while the latter by its strength and tension prevents it from being 

 hurt by any external pressure. 



But of all the contrivances by which insects in this state are secured 

 from their enemies, there is none more ingenious than that to which the 

 May-flies (Trichopterd) have recourse for this purpose. You have heard 

 before that these insects are at first aquatic, and inhabit curious cases 

 made of a variety of materials, which are usually open at each end. 

 Since they must reside in these cases, when they are become pupae, till 

 the time of their final change approaches, if they are left open, how are 

 the animals, now become torpid, to keep out their enemies ? Or, if they 

 are wholly closed, how is the water, which is necessary to their respira- 

 tion and life, to be introduced ? These sagacious creatures know how to 

 compass both these ends at once. They fix a grate or portcullis to each 

 extremity of their fortress, which at the same time keeps out intruders 

 and admits the water. These grates they weave with silk spun from 

 their anus into strong threads, which cross each other, and are not soluble 

 in water. One of them, described by De Geer, is very remarkable. It 

 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 circumference, 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.^ 



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 

 (Phalctna L.) — amounting in this country to more than a thousand spe- 

 cies — with few exceptions, are all night-fliers. And a considerable pro- 

 portion of the other orders — exclusively 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 (^Orectocheilus villosus), 

 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.^ Many larvcB 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 puUa and Nycierobius, whose 

 proceedings have been before described. The caterpillar of another moth 

 {Noctua subterranea F.) never ascends the stems of plants, but remains, 



» Reaum. v. 100. * Reaum. iii. 170. De Geer, ii. 519. 545. 



' Ann. Soc. Ent. de France, ir. bull. Ixxx. 



