14 
there are at least four distinct species,—lay their eggs 
very frequently either just below the surface of the 
ground, or, if possible, at the base of the bulb. I have 
found that if onion bulbs are earthed up from time to 
time the flies are thus obliged to lay their eggs hap¬ 
hazard, on the leaves or ground, or at least not in such 
a position that the maggots, if they hatch at all, can 
make their way to their food, and this prevents very 
much of the attack, and agrees well with the plant - 
growth. 
Larva .— In this country the conditions which are 
most favourable for the hatching of the larvae of the 
greater part of our insects are moderate warmth, to¬ 
gether with some degree of moisture. We may see this 
from the larger proportion of eggs which hatch during 
the warm season of the year ; but heavy rain, and espe¬ 
cially rain following after a term of warm, dry weather, 
is most destructive to larvae which are exposed to it, 
such as various kinds of caterpillars of moths, butter¬ 
flies, and some kinds of sawflies. It is stated in ‘ Farm 
Insects ’ by John Curtis that most caterpillars are 
purged by wet, but that some (the caterpillars of the 
Large White Cabbage Butterfly, for instance) do not in¬ 
variably suffer. Mention is made of caterpillars of this 
kind, which had grown rapidly during some hot dry 
days, being found on the afternoon succeeding a rainy 
night dead and empty, the lax skins merely containing 
some fluid. In the attack of the Silver Y Moth (the 
Plusia gamma) in 1879, the ravages of the caterpillars 
were recorded as being checked at Maldon, in Essex, by 
the drenching rains; and in the case of the Silkworm, 
the bad effect of wet food to their caterpillars is well 
known to the growers. Some sawfly-caterpillars are 
similarly affected. Those of the Pine Sawfly are very 
susceptible of wet and cold, and, though the black cater¬ 
pillars of the Turnip Sawfly will bear a shower, they are 
sometimes cleared in great numbers by the heavy down¬ 
pour of a thunderstorm ; also it is stated that rains 
are singularly destructive to these caterpillars by 
