76o LIFE : OUTLINES OF GENERAL BIOLOGY 



relation to the environmental conditions in which each organism has 

 to live. Thus it is obvious that it is advantageous for animals living 

 in the rough-and-tumble conditions of the seashore to have a pro- 

 longed larval period in the relatively safe open sea. 



Illustrations. — In many animals there is a disproportionately 

 prolonged youthful period, to a discussion of which we shall return 

 in considering larvae. What emerges from the egg of a locust, grass- 

 hopper, cockroach, or earwig is practically a miniature of the 

 parent, though there may be some differences as regards colour and 

 wings; but what emerges from the egg of a Mayfly is an aquatic 

 larva with little resemblance to the adult. This larva feeds, grows, 

 and moults, but may remain larval for two, three, or perhaps four 

 years. It then changes into a "nymph", and then after a final moult 

 on the bushes by the side of the stream — a moulting unique, since 

 it occurs after the wings have been formed — it puts on the finished 

 adult characters. But this aerial Mayfly is now specialised almost 

 only for reproduction, and dies after an ecstatic evening or two, 

 without a single meal! Truly diagrammatic is the life-curve of one 

 species which has its full-grown life telescoped into one brief hour. 

 It is not even an ephemerid; and yet when the complete curve is 

 kept in view, it cannot be called short-lived. 



Among insects there are many instances of prolonged pre-adult 

 stages, and the lengthening out may be in the embryonic, the larval, 

 or the pupal chapter, or even in all of them. In a hive-bee the egg 

 usually takes about 3 days to hatch; the worker-grub is fed and 

 grows for about 6 days; the pupal stage, behind a waxen screen, 

 extends over 12 days; the winged insect is ready to come forth 21 

 days after the egg was laid; and the worker-bee, after an indoor 

 apprenticeship of about three weeks, issues forth as a forager, 

 exhausting herself in about a month, or at the most six weeks. 

 Some authorities estimate the average summer life of a winged 

 worker-bee at 42 days, which is only twice as long as the antecedent 

 period of preparation. 



In some cases, as in butterflies and moths, there is a prolongation 

 of the pre-adult phase or phases, throughout the winter, often in 

 an inactive state. A notorious case is that of the North American 

 "seventeen-year Cicada" {Cicada septemdecim) , which spends all that 

 time as an underground larva, and then enjoys a brief and noisy 

 aerial existence. Over the States as a whole there is a representation 

 of Cicadas almost every year, but the individual life-cycle is one of 

 seventeen years. Large numbers appear with some regularity every 

 seventeen years, but in other localities, especially to the south, the 

 usual interval is thirteen years — an interesting time- variation which 

 well illustrates our general idea that there are, as it were, elastic 

 arcs on the life-curve. The difference between the 17-year and 

 13-year varieties of the species may perhaps be correlated with 



