Egg-masses 155 



live iu water, it is convenient that tlie egg-chains should 

 be laid in water, and further that they should float at the 

 surface, where they can be freely supplied with air, and 

 run no risk of being smothered by silt or organic refuse. 

 If the water were stagnant, the eggs might float free, as 

 the egg-raft of the gnat does, but the eggs of Chironomus 

 dor sails are laid in slow streams, and must be secured, 

 lest they should be swept away, and perhaps lodged in 

 some unsuitable place, or even carried out to sea. The 

 eggs of this species are therefore invested by a gelati- 

 nous envelope, which swells out, the moment it touches 

 the water, into an abundant transparent mucilage, and 

 the whole mass is moored to some fixed object by twisted 

 cords. The mucilage has its special uses : it makes the 

 egg-mass slippery, so that birds or insects cannot grasp 

 it : moreover, it spaces the eggs, so that each is well 

 exposed to the sunlight and air ; lastly, it keeps oft' the 

 attacks of the water-moulds (Chytrideae and allied Oomy- 

 cetes), which abound in water and on the surface of 

 decaying plants, or devour the substance of living insects 

 and fishes. It may be that the mucilage of the egg- mass 

 has some antiseptic property, for it remains unchanged 

 by parasitic growth or putrefaction long after the eggs 

 have hatched out. 



During the summer months the egg-masses of Cliiro- 

 nomus dorsalis are readily found. It is not indeed easy to 

 detect them on the weedy bank of a stream, but the fly 

 often lays them on the edge of a stone fountain, or in 

 a watering trough by the side of a road. If an egg-mass 

 is dipped into boiling water, the way in which it is moored 

 becomes evident. An enclosed double cord, previously 

 invisible, now becomes opaque enough to be examined by 

 a lens (fig. 1 16, B). It passes through the cylinder in a series 

 of loops, and then returns in as many reversed and inter- 

 twined loops, so as to give the appearance of a lock-stitch. 



