Aprh. 1, 1896,] 



KNOWLEDGE. 



83- 



any length, as also the many forms of domestic filter 

 which are used for effecting the same object ou a small 

 scale. Plagge, some years ago, made an elaborate 

 examination of nearly all the domestic filters then in use, 

 and contributed the results of his investigation to the 

 German Association of Naturalists and Physicians. His 

 results show the uusatisfactoiy nature of the majority of 

 these filters, and his conclusions have been confirmed by 

 the recent careful study of similar filters in use in this 

 country by Drs. Sims, Woodhead, and Wood. 



THE WINTER LIFE OF INSECTS.-I. 

 By E. A. BuTLEB, B.A., B.Sc. 



IF we stroll through the woods on some bright summer 

 day, we find the air full of sounds — an indistinct hum 

 that betokens the presence of innumerable insects ; 

 and every step we take disturbs multitudes of flying 

 or crawling things which retreat in aU directions 

 before us. But if we go to these same woods in winter, 

 silence reigns around, and scarcely a vestige of insect life 

 appears in whatever direction we wander. This contrast, 

 though a perfectly familiar observation, can hardly fail to 

 suggest to the thoughtful mind inquiries as to what hnk 

 of connection may exist between these successive annual 

 swarms, separated from one another, as they seem to be, 

 by Icng periods during which there is a dearth of life. 

 Admitting, as we must do, that all the living beings of any 

 one period are the direct descendants of those of a previous 

 one, only one or other of two explanations of the regularly 

 recurring appearance of these summer hosts is open to us. 

 Either these creatures are the same individuals that we 

 saw the year before, which have been spending the winter 

 in some sort of retirement, or they belong to the next 

 generation, and are the progeny of the preceding year's 

 population. In this latter case, however, equally with the 

 former, since no break can have occurred in the continuity 

 of life, there must have been some form in which these 

 little beings have continuously existed since last we were 

 struck with the multiplicity of their activities. 



There is truth in both these hypotheses. The particular 

 specimens of insect life that enliven the woods, fields and 

 hedgerows during any given season are in some cases the 

 same individuals that were to be met with the year before, 

 and in others they are members of the next generation. 

 The exact duration of an insect's life is, in very many cases, 

 an unknown quantity, and often the imcertainty is greatest 

 in connection with that part which is spent after the 

 insect has passed tlirough its metamorphoses and attained 

 its perfect form. The exact time occupied by the pre- 

 liminary stages may be known with great accuracy, but 

 after the insect has been conducted through its various 

 changes, information often fails as to how long it will 

 continue to exist in the form it has by that time acquired. 

 And the results obtained by keeping insects in confine- 

 ment are not altogether reliable as to the usual duration 

 of life, since their cuxumstances and surroundings are more 

 or less unnatural, and while in some cases, no doubt, certain 

 aids to longevity are absent, in others the risks to life and 

 limb which would be encountered in the wild state are almo.st 

 indefinitely diminished. Sir John Lubbock kept some 

 queen ants alive for upwards of eight years, and Dr. Sharp 

 records of a pair of large wat:r beetles (not British) that 

 the male lived with him for two years, and the female 

 for five. Esper, again, kept the common water beetle 

 (Dytiscus martjinalis) (Fig. 1) for three and a half years, 

 and Koesel a rose beetle {Ceto7iia aurataj (Fig. 2 1 for nearly 



Fig. 1. — Common 

 ■\Vater Beetle (Vy. 

 tiscus margviialis) . 



three years. An English entomologist of the last century 



kept a cellar beetle (Blaps) for upwai'dsof three years, after 



he had made four attempts to drown it in spirits of wine, and 



this record of hardiness and longevity 



was brought to a close, not by any 



failure of the insect's powers, but by 



the carelessness of a domestic, who 



allowed it to escape. These instances 



suffice to show that, in certain species 



at least, there is enough vitality in the 



perfect insect to carry it through a 



much longer period than has usually 



been believed possible, and to open 



up the chance of meeting with the 



same insect, even in nature, during 



more than one season. And fitrther, 



the suggestion is obvious that, if such 



longevity does not occur in nature, 



imtoward external circumstances may often be looked to 



as the causes producing death, rather than internal decay 



and senility. 



It may safely be assumed that the chief purpose of the 

 life of the adult insect is to secure the perpetuation of its 

 species. The main business for which the creature is 

 adapted in the complex economy of nature, whether that 

 business be the repression of superabundant vegetable 

 or animal life, or the removal of waste matters, will in 

 very many cases have been performed during its pre- 

 liminary stages, and then there remains for the adult 

 little more than the reproduction of its kind ; hence there 

 would appear to be no particular reason for any great 

 prolongation of its life after it has mated, or, if a female, 

 after it has laid its eggs. Parental care for offspring after 

 the laying of the eggs not being customary amongst msects, 

 the final function of their existence will usually have been 

 performed when provision has been made for the next 

 generation. Moreover, the function of reproduction is 

 so exhaustive of the natural energies that death often 

 follows immediately after it. Indeed, in some species the 

 adult insect is so entirely devoted to this fimction, that no 

 provision exists for its taking food ; its very mouth organs 

 have become aborted, and under these circumstances, of 

 course, a long life of activity would be an impossibility. 

 It seems highly probable, therefore, that the act of 

 reproduction is usually the last in the insect's life, and 

 that the duration of its life will be largely determined by 

 the date of that act ; hence, if circumstances prevent 

 reproduction from taking place at the usual time, life will 

 stand a good chance, if the insect has the power of taking 

 food, to be prolonged until it has taken place. 



Now it is not possible to name a single month in the 

 year which may not witness the deposition of eggs, on the 

 part of some species or other ; and hence it follows that 

 the absence of insect life in the winter is more apparent 

 than real. Perfect insects may be found all the year 

 round, provided only we know where to look for them. 

 But while in summer they obtrude themselves on our 

 notice, in the winter they need a great deal of searchmg 

 for. I propose to point out, so far as possible, in what 

 condition the various kinds of insects pass the winter and 

 where they may be foimd ; but the many gaps that still 

 exist in the knowledge that has been acquiied of life- 

 histories render it impossible to do this otherwise than 

 imperfectly. This imperfection, however, may perhaps 

 stimulate some of the readers of Knowledge to conduct 

 observations with a view to increase the existing stock of 

 ascertained facts. But before proceeding to our subject in 

 detail, it will be well once more to emphasize the fact, 

 obvious though it may be, that every single insect we meet 



