INSECTS 



399 



Among freshwater forms the Great Black Water- Beetle 

 {Hydrophilus piceus) is interesting in regard to its egg-laying 

 arrangements. The hinder part of its bocy is provided with 

 spinning glands, by the aid of which it constructs a pear-shaped 

 silken case, within which the eggs are deposited, and which is 

 attached to the leaf of a water-plant (fig. 918). 



Scale-winged Insects [Lepidoptera). — The Butterflies and 

 Moths which make up this order are often so attractive in 

 appearance that there has never been a lack of naturalists willing 

 to study them, and a vast amount of information has been col- 

 lected regarding their struc- 

 ture and habits. The se- 

 quence of Ggg, caterpillar, 

 pupa or chrysalis, and imago, 

 is familiar to everybody. 



The large White or Cab- 

 bage Butterfly iyPieris bras- 

 siccs, fig. 919) furnishes a 

 common and instructive illus- 

 tration. The female lays 

 clusters of bright yellow 

 eggs on the under side of 

 the leaves of cabbages, tur- 

 nips, and allied plants. The 

 external surface of the &<s,<y is 

 sculptured in a characteristic 

 manner. The caterpillars are gregarious, and when fully grown 

 are bluish-green above and yellow below, with a yellow streak 

 down the middle of the back, a series of large black spots along 

 either side, and a sprinkling of black dots over the body. These 

 colours make the larvae conspicuous objects, especially as a number 

 are associated together, and they are regarded as an illustration of 

 warning coloration (see vol. ii, p. 301). After attaining its full 

 size the caterpillar attaches itself by the tail to some firm object, 

 spins a silken girdle for the front part of its body, casts its skin, 

 and passes into the chrysalis stage. The observations and ex- 

 periments of Poulton and others have shown that the colour 

 harmonizes with the surroundings, varying from green to almost 

 black, and this is interpreted as variable protective coloration 

 (see vol. ii, p. 289). There may be several broods during the 



Fig. 919. — Cabbage Butterfly {Pieris brassiccE). i, Adult 

 female; 2, eggs; 3, caterpillar; 4, chrysalis. 



