ADAPTATIONS TO HAUNTS AND SEASONS 279 



woven cocoons, attached to the water-plants where they fed 

 and breathed as larvae, and although the adult beetles emerge 

 from the pupal cuticle in autumn, they remain in their 

 submerged cocoons through the winter. The dense wall 

 of the cocoon, water-tight and air-tight, is formed largely 

 of silk derived from modified salivary glands, and partly, 

 according to Boving, of the secretion of skin-glands with the 

 addition of the ejected and hardened contents of the food- 

 canal. MacGillivray states that the lar\'a works at its cocoon 

 \vith the spiracular spines embedded in the plant-tissues 

 from whose air-spaces the cocoon becomes inflated ^^ith 

 air when complete. Then the lar\-a withdraws from the 

 plant stem or root and turns round before casting its cuticle. 

 Through the scar in the stem or root abundant air-supply 

 can pass from the air-spaces to provide for the breathing 

 of pupa and beetle until the spring season arrives when it 

 emerges into the upper air. The ventral surface of the body 

 being covered with fine hairs, a water-film surrounds it, so 

 that the beetle has an air-bubble over the spiracles as it 

 rises at last into the atmosphere, after its prolonged immature 

 existence under water. It has already been mentioned that 

 some of these insects may descend again beneath the surface- 

 film in order to feed and lay their eggs on the submerged 

 aquatic plants. 



The short hairy clothing of the Donaciine beetles affords 

 an example of a condition common in many aquatic insects 

 for enabling them to crawl or dive under water without 

 the actual surface of the cuticle and its open spiracles 

 becoming wet. The tips of such hairs are too close together 

 to allow water to penetrate between them ; as the insects 

 pass beneath the surface the film on which they press 

 remains unbroken, and when the insect is submerged a 

 layer of air, evident from its silvery sheen, extends over at 

 least a portion of its body ; thus it takes with it into the 

 water enough of the atmosphere to supply for a while its 

 needed ox}^gen. The tension of the surface-film of water, 

 well known to students of physics, renders possible some 

 of the most remarkable adaptations of aquatic insects ; 



