226 



KNOWLEDGE. 



[October 1, 1894. 



of its mouth organs and the consequent loss of the power 

 to take food and sustain active life. Hence arises the 

 necessity for special means of concealment and protection, 

 that the race may not be exterminated when placed at so 

 great a disadvantage with regard to its ever watchful and 

 never resting foes. The necessary concealment or pro- 

 tection is afforded in many ways, occasionally by the use 

 of foreign materials, but much more frequently by the 

 production of a substance from within the creature's own 

 body to serve as a cloak of invisibility or a chamber of 

 safety. But as, in due course, deliverance from this 

 chamber becomes necessary, it is equally imperative that 

 another product should be elaborated for this purpose where 

 mechanical means are not available. 



Then, again, insects are usually very small animals in 

 comparison with most of their assailants, and with small- 

 ness of size there is, of course, associated a corresponding 

 helplessness against the larger and more powerful. And 

 though in many cases their speed of locomotion is con- 

 siderable when we take into account the size of the 

 creatures concerned, yet the rate at which larger distances 

 are covered is extremely slow in comparison with that 

 exhibited by their pursuers, and hence escape by simple 

 speed is usually out of the question ; whence arises the 

 existence of numerous secretions rendering them distaste- 

 ful, or even physiologically hurtful as articles of food. 



The development of the social instinct again, involving 

 as it does the association of large numbers of the same 

 species in one spot, necessitates the provision of suitable 

 accommodation for the rearing of large quantities of 

 young. And though this accommodation is frequently 

 provided to a large extent by means of external and 

 foreign matter, we see in the waxen cells of bees a product 

 of the internal physiological processes of their own bodies 

 usfd for this purpose. And even when the insects are not 

 social, but only gregarious, there is, as we have just seen, 

 provision for the large numbers of eggs produced, the 

 provision on the part of the mothers of cases or sacs 

 which shall supply the place of that maternal care and 

 supervision which in the case of higher animals would 

 probably be rendered. 



The shortness of insect life is another important point. 

 As few insects live for more than one season, and the 

 period of activity of some of them lasts over no more than 

 a few weeks, separated by an interval of many months from 

 the next period of activity of the species, the conthiuance 

 of the race is absolutely depfndent upon the meeting and 

 mating of suitable pairs during those few weeks. When 

 the species is gregarious, or so abundant that large num- 

 bers are sure to be found within easy distance of each 

 other, there is not likely to be much difficulty on that 

 score. But when the species is solitary or scarce, and 

 specimens occur only at considerable distances from one 

 another, special aids seem to be needed to ensure the 

 necessary pairmgs, and it may be that it is for such a 

 purpose that some of the characteristic odours of insects 

 exist. 



Finally, in the case of the honey-dew of the aphides, 

 none of the above heads seem suitable as an explanation 

 of its existence. Here we have a material, not produced 

 on certain special occasions, times of greater danger than 

 usual, but continuously elaborated throughout life. A 

 glance at the accompanying plate will show that the cor- 

 nicles from which the drops issue are present in the larval 

 forms as distinctly as in the nymphs or the adult_ insects. 

 This fact, coupled with the above-mentioned difficulty of 

 ranking honey-dew under any of the heads just enumerated, 

 and with the statement made in a former paper, that no 

 connection has been traced between the cornicles and any 



special glandular structure, lends support to the theory 

 which makes it an excretion rather than a secretion ; or, 

 in other words, a substance elaborated to be got rid of 

 rather than to be used. There is, further, the very excep- 

 tional circumstance, that it is a substance not merely got 

 rid of by one kind of insfct, but also eagerly sought after 

 and used as food by another ; and this fact is rendered 

 more striking still by the immense differences, both in 

 structure and life-history, that exist between the insects 

 thus associated as producers and consumers, the aphides 

 and the ants. 



PHOTOGRAPHS OF THE MILKY WAY AND 

 NEBULA. 



By A. C. E.\NYAED. 



WE are indebted to Mr. W. E. Wilson for another 

 photograph of the Hercules cluster, taken 

 with an exposure of only fifteen minutes. 

 The photographic action has not gone far 

 enough to blot out and drown with nebulosity 

 the prominence forms and strings of stars shown in the 

 Henry and Lick photograplis, reproduced in the April and 

 May numbers of Knowledge for 1893. The sharpness 

 and clear definition of thtse details, shown in Mr Wilson's 

 photograph, speaks very highly for the optical perfection 

 of bis reliictor, which was made for him by Sir Howard 

 Grubb. Even the photographic reproductions, which lam 

 able to put before the readers of Knowledge, will well repay 

 a cartful study and comparison with the longer exposed 

 photographs reproduced m the last number of Knowledge, 

 as well as with the Henry and Lick pictures. It seems to 

 me, from a study of the photographic prints sent by Mr. 

 Wilson, that there is ample evidence of more than one 

 dark promintnce-like structure, indicating that light- 

 absorbing material, as well as luminous matter, is ejected 

 in streams from the central region of the cluster, and that 

 in rushing outward it experiences resistance. The photo- 

 graph also shows the nebulous ligatures joining lines of 

 stars, referred to in the last number, and confirms the 

 suspicion that many of the star discs are not circular, but 

 are elongated in the direction of the chain of stars. They 

 appear more like patches of brighter nebulosity with a 

 sensible area than like star discs. 



The photographs of the dumb-bell nebula, also sent by 

 Mr. Wilson, are well worthy of study. The dark gaps in 

 the nebulosity are as interesting as the bright structure. 

 This nebula seems to have a bright stellar nucleus, and a 

 curious stratum of brighter nebulosity at the outside. 

 With longer exposures the dumb-bell form is lost, and the 

 nebulosity extends so as to fill an elliptic area. 



We are indebted to Dr. Max Wolf, of Heidelberg, for 

 the photograph of the region of the Milky Way about 

 /3 Cassioptife. The silver prints from the original negatives 

 which he has sent over show some enormous dark branching 

 structures outlined by stars. The collotype reproduction 

 does not show them nearly as well as the silver prints, but 

 there can be no doubt about their existence. 



Notices of 33oofts. 



: • 



A Xaturalist on the Piviil or in the Juvf/le. By E. H. A. 

 (Thacker & Co., London, 1894.) E. H. A. has already 

 attracted very favourable public notice by his lively books, 

 "The Tribes of my Frontier," and "Behind the Bungalow," 

 which introduce the reader to the human surroundings of 

 an Anglo-Indian's life. In this book he brings before us 



