428 MEANS OF DEFENCE OF INSECTS. 



become torpid, to keep out their enemies ? Or, if they are wholly closed, 

 how is the water, which is necessary to their respiration 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, de- 

 scribed 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 re- 

 gularly 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 compart- 

 ments, 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 (P//#- 

 Icena 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 exclusively oFthe 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 

 congenei-s, 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. 8 Many larva 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 Nycterobius^ 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. 



I am now to lay before you some examples of the contrivances, requiring 

 skill and ingenuity, by which our busy animals occasionally defend them- 

 selves from the designs and attack of their foes. Of these I have already 

 detailed to you many instances, which I shall not here repeat ; my history, 

 therefore, will not be very prolix. I observed in my account of the so- 

 cieties of wasps, that they place sentinels at the mouth of their nests. 

 The same precaution is taken by the hive-bees, particularly in the night, 

 when they may expect that the great destroyers of their combs, Galleria 

 mellonella and its associates, will endeavour to make their way into the hive. 

 Observe them by moonlight, and you will see the sentinels pacing about 



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

 8 Ann. Soc. .Ent. de. France, iv. bull. Ixxx. 

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



