I20 INJURIOUS AND USEFUL INSECTS 



off to lay her eggs, while the male rejoins his comrades. 

 Since the females remain so short a time in the swarm, it is 

 not surprising that a sweep of the net, which captures hundreds 

 of males, should often contain not a single female. But when 

 the swarm is much agitated by wind, pairing becomes difficult, 

 and a number of females may be forced to remain un- 

 jnated. 



The conditions of egg-laying are peculiar, and call for many 

 special provisions. The eggs must be numerous, as the larva 

 runs many risks ; they must be laid in water, although the fly 

 cannot enter the water without being drowned; they must 

 float at the surface, lest they should be overwhelmed by the 

 refuse of a foul stream ; they nmst be made fast, so that they 

 do not drift into some quite unsuitable part of the stream, or 

 even into the sea. Floating eggs are exposed to the attacks of 

 hungry creatures of various kinds, such as birds, predatory 

 water-insects, and fishes. These difficulties, which are many 

 and real, have been surmounted in a singularly interesting 

 way. The eggs are coated with a gelatinous envelope, which 

 swells out the moment it reaches the water, into an abundant 

 transparent mucilage. This mucilage answers more than one 

 purpose. In the first place it makes the eggs so slippery that 

 birds or insects cannot grasp them. It also spaces the eggs, 

 and enables each" to get its fair share of air and sunlight. The 

 gelatinous substance appears to possess some antiseptic property, 

 which prevents water-moulds from attacking the eggs, for, long 

 after the eggs have hatched out, the transparent envelope 

 remains unchanged. The egg-mass of a harlequin-fly forms a 

 transparent cylindrical rope, nearly an inch long, which floats 

 at the surface of the water. The eggs are arranged just below 

 the surface of the egg-mass in loops, which wind to right and 

 left alternately. The whole egg-rope is moored to the bank by 

 a thread, which passes along the middle in a series of loops ; 

 and returns in as many reversed loops, so as to give the 

 appearance of a lock-stitch. The threads are tough, and 

 perfectly invisible in the natural state, owing to their trans- 

 parency, but they can be made apparent by dipping the egg- 

 rope into boiling water, when they turn white. Thus the 

 whole mass, containing nearly a thousand eggs, is firmly 

 moored, yet so moored that it rises and falls with the stream. 

 The eggs get all the sun and air which they require, and 



