THE INSURGENCE OF LIFE 147 



the tide rises it retreats far into crevices or into the 

 sand. The whitish hairs on its body hold a supply of air, 

 which may last for 4J days. There are also two British 

 parasitic gall-flies that occur at high- water mark among the 

 sea-weed of the jetsam. 



Aquatic Insects. — The adaptations of aquatic insects 

 form a well-nigh inexhaustible theme. Let us take a 

 couple of instances from Dr. Boring's account of the larvae 

 of the Donaciinse, a sub-family of the Chrysomelids, or leaf- 

 beetles. The larvae puncture the roots of water-plants, 

 such as pond- weed and Sparganum, and feed on the exu- 

 ding sap. In making the hole, by means of the cutting 

 mandibles, the neatest possible contrivance comes into 

 operation. The first segment of the thorax sUps forward 

 against the plant and within the water-tight compartment 

 thus formed, the head works freely and the sap is kept 

 from adulteration with water and debris. 



The adaptation for breathing is not less striking, for the 

 larvae manage to tap the stores of air in the intercellular 

 spaces of water plants. A hooked breathing pore or 

 spiracle at the end of the abdomen is plunged into the 

 tissue of the plant and the air finds its way (in a somewhat 

 intricate fashion) into the breathing-tubes or tracheae of 

 the insect. Similarly, after the larva has enveloped itself 

 in a secreted cocoon, it actually bites a hole, or more 

 than one, at the bottom and estabhshes connexion with 

 the air spaces of the root to which it is attached. In this 

 way it secures a supply of air during its pupal period ! 



Against the Grain. — It seems to be part of the 

 Amphibian constitution to have an antipathy to salt — 

 a small quantity being often fatal. Referring to the absence 

 of Amphibians from strictly oceanic islands Darwin pointed 



