336 



INTRODUCTION TO ZOOLOGY 



CHAP. 



are narrow, spindle-shaped, and dark brown, and they are 

 laid in irregular overlapping clusters on the under sides of 

 the leaves of water-plants frequently the water plantain 

 which rise a little above the water level in 

 some stagnant pond or ditch (Fig. 257). 

 The little larva when it hatches makes its 

 way rapidly down into the water, appar- 

 ently pulling itself along by the well- 

 L H| developed pair of appendages on its head 



(Fig. 258, p). When it reaches the water, 

 it floats at the surface, lying at first hori- 

 zontally, but ultimately taking up a more 

 FIG. 258. The Head or less vertical position, head downwards, 

 of a very^ young it i s supported in this position by the 

 beautiful circlet of hairs that surrounds the 

 end of the tail. These hairs spread out, 

 forming a little basin filled with air, and the surface tension 

 between the water film and the hairs is sufficient to support 

 the body. Into the little air -basin open the spiracles at the 

 tip of the tail. The larva is at first white and transparent, 

 but when full grown it may be 2 inches long and is yellowy- 

 brown in colour. It hangs almost motionless, feeding on the 

 microscopic organisms which pass in the water, driving them 

 into its mouth by the movements of its palps. If alarmed, the 

 larva quickly draws its tail circlet of hairs together, enclosing 

 a little air -bubble in their midst, and dives down into the 

 water with a wriggling movement; there are no traces of limbs 

 or special appendages of 

 any sort to aid in loco- 

 motion. On ceasing the 

 attempt to descend, it 

 floats up to the surface 

 again. Although the larva 

 breathes only through its 

 tail, there is a pair of 

 spiracles on each segment 

 of the body except the 

 second and third. These spiracles, however, are not func- 

 tional in this stage. 



When the time comes for the change to the pupa, the 

 larva may leave the water, or it may remain floating at the 



FIG. 259. The Pupa of Stratiomys 



chamaeleon, floating. 



The dark body of the fly can be seen in the front 

 part of the otherwise empty skin. 



