RELATIVE DURATION OF LIFE. 
141 
food-plant is another factor which prolongs the life of the adult, since the 
mother insect will remain alive until eggs are laid on the food-plant unless 
this period is so long as to exhaust her vitality. 
What terminates an insect’s life ? If we consider the insects which 
escape their foes, which do not die of injury, of parasites or of disease, 
but which die a natural death, what brings about the cessation of life ? 
Speaking very broadly, the full exercise of the natural functions of re¬ 
production brings a speedy end, perhaps from exhaustion, perhaps from 
a lack of vitality now that there is no further object in life. The locust 
dies, if a male after coupling, if a female after the deposition of all the 
eggs, though food may be abundant and the conditions apparently 
suitable for further life. The moth dies after mating or laying eggs, and 
the life of many moths is limited to one or two nights if reproduction 
is effected, though it may be much extended if mating and egg-laying 
be not possible ; and this is true even of moths that cannot feed and in 
which the alimentary system is wholly undeveloped. 
In estimating the natural life of an insect, we have to consider the 
time required to build up the tissues of the larval or nymphal as well as , 
those of the subsequent imaginal form, the time required to reproduce, 
as w^ell as the conditions of food-supply and temperature under which 
life is carried on. For many, the conditions of food-supply and tem¬ 
perature are such that a yearly period covers the whole life, there being 
one brood yearly. For others, one active season is not sufficient for the 
larval form to lay up sufficient nourishment to provide for the tissues 
of the imago ; or this may be possible during the limits of a season or two 
seasons, but the processes of transformation cannot be completed in time 
to allow of the imago to emerge, mate and lay eggs at a favourable sea¬ 
son and before the rigours of winter or drought prevent the imago from 
providing for the young. Thus we get a two-year or a three-year period, 
the whole life from egg to egg occupying multiples of one year. In rare 
cases (so far as known) this period may be peculiarly long and the Cicadas 
are notorious in this respect; the 17 years of Tibicen septindecin, and 
the 13 years of Cicada tredecin , both American insects, are notorious in¬ 
stances. Turning to shorter-lived insects, we find for instance the two- 
brooded butterflies, in which there is one quick brood in the rains, and 
one longer brood which persists in some form through the cold and dry 
weather till food is again available on the coming of the rains or perhaps 
at the opening of the buds in spring. From these, a large class probably, 
we come to those which have several broods in the limits of the hot 
weather and rains and which have one longer brood, with a long inactive 
period in the colder weather. The active periods in these cases are the 
same, but one brood must pass through the long inactive period. 
We come finally to normally very short-lived insects such as many 
Diptera, in which the egg, the larval, the pupal and the imaginal life 
are contained within perhaps 14 days, the actually known shortest 
being about 7 days. For these insects life may be long, but given the op- 
