YOUTH IN BIRDS AND LOWER ANIMALS 55 



of common flies such as the blow-fly and the house-fly is a little 

 longer. The blow-fly hatches out in twenty-four hours, the larva 

 takes a fortnight to grew, whilst the metamorphosis within the 

 ipupa case takes a fortnight m warm weather, and much longer 

 when it is cold. The normal hfe of the adult fly is from a few days 

 to a few weeks, or in specially favourable circumstances, a few 

 months. The length of the larval hfe of butterflies and moths 

 varies according to the size, the habits and the weather, and as in 

 extreme cases the hfe of the adult may last a good many months, it 

 is possible that the total cycle may sometimes extend a little over a 

 year. Amongst bees, the larval life and the metamorphosis occupy 

 at most a few weeks, whilst the hfe of the adult is relatively longer. 

 Worker bees never live beyond the year in which they are produced ; 

 whilst the hfe of drones may be only a few days, and is never more 

 than a few months, as towards the end of the season, when honey is 

 getting scarce, they are driven out of the hive to perish. Queen 

 bees may live from two to five years ; they are fed and cared for by 

 the workers, and their confinement to the hive after the nuptial 

 Iflight preserves them from the vicissitudes of the weather. 



The instances that I have given do not show any great eccentricity 

 in the distribution of the total duration of life between the youthful 

 'and the adult stages. The proportion between the duration of 

 youth and of adult life certainly varies, but not much more than it 

 varies in higher animals, and we do not know enough about the 

 physiology of insects to assign reasons for these different durations, 

 and still less are we able to draw parallels between the lengths of 

 the period of youth and the degrees of intelligence. The mental 

 processes of insects and their modes of communication with the 

 exterior are so unlike our own that our attempts to discriminate 

 between instinct and intelhgence must be the most casual guesses. 

 In very many insects, however, the disproportion between adult 

 life and larval life is so great that adult hfe appears to have been 

 reduced merely to the time required for reproduction. Many adult 

 moths and butterflies have no mouths and do not feed. The males 

 live only long enough to meet and fertilise the other sex, and the 

 females live a little longer, apparently only because they have to 

 seek out food-plants or places specially suitable for the larvae which 

 will hatch out from the eggs they lay. The eggs of the mayflies 

 are dropped into the water and in a few months hatch out into 

 creeping campodeiform larvae. These live, according to the species, 

 from six months to three years in the water, and then come up to the 



