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 cireumference, 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.! 
Under this head I shall call your attention to another circumstance that 
sayes 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 (Pha- 
Jena \..) — 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 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 /arve 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 pullaand 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.? 
The habitations ofinsects 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. 
Tam now to lay before yousome 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. [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, Gallena 
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 
1 Reaum. iii. 170, De Geer, ii. 519. 546, 
& Ann. Soc. Ent. de. France, iv. bull. xxx, 
© Fab. Ent. Syst. Lm. iii. 70. 200. 
