SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 351 
inch in length, may readily be found on the edges of a stone fountain 
in a garden, or in a water trough by the side of the road. The 
eggs are arranged upon the outside of the rope in loops, which bend to 
right and left alternately, forming sinuous lines upon the surface. 
Kach egg rope 1s moored to the bank by a thread, which passes through 
the middle of the rope in a series of loops, and then returns in as many 
reversed and overlapping loops, so as to give the appearance of a lock 
stitch. The thread is so tough that it can be drawn out straight with 
a needle without breaking. If the egg rope is dipped into boiling 
water the threads become apparent, but in the natural state they are 
invisible, owing to their transparency. The mucilage is held together 
by the threads inter-woven with the mucilage. The loops can be 
straightened without injury until the length of the rope is almost 
doubled. If stretched beyond this point the threads become strained 
and do not recover their original shape when released. By means of 
these threads, firmly inter-woven with the mucilage of the egg rope, the 
whole mass of many hundreds of eggs is firmly moored, yet so moored 
that it floats without strain, and rises or falls with the stream. The 
eges get all the sun and air which they require, and neither predatory 
msects, nor birds, nor water molds, nor rushing currents of water, 
‘an injure them. 
The eggs of the ecaddis-fly are laid in larger ropes, which, in some 
species, are very beautiful cbjects, owing to the grass-green color of 
the eggs. The egg raft of the gnat, which has often been described, 
is well suited to flotation in stagnant water, and is freely exposed to 
the air, a point of unusual importance in the case of an insect which in 
all stages of growth seems to need the most efficient means of respira- 
tion, and whose eggs are usually laid in water of very doubtful purity. 
The lower or submerged end of each egg opens by a lid, and through 
this opening the larva at length escapes. 
The eggs of water-haunting insects are in many ways particularly 
well suited for the study of development. The eggs of Chironomus, 
for instance, can always be procured during the summer months, They 
are so transparent as to admit of examination under high powers of 
the microscope as living objects, and as they require no sort of prep- 
aration, they may be replaced in the water after each examination to 
continue their development. This saves all trouble in determining 
the succession of the different stages, a point which usually presents 
difficulties to the embryologist. The whole development of the egg of 
Chironomus is completed in a few days (three to six, according to tem- 
perature), and it is therefore an easy matter to follow the process 
throughout with the help of three or four chains of eggs. 
When the larva are hatched and escape into the water, new difficul- 
ties arise. Some have to seek their food at the surface of the water, 
and must yet be always immersed, others live upon food which is only 
to be found in rapid streams, and these run serious risk of being swept 
