Mat 1, 1894.] 



KNOWLEDGE 



103 



low as HiiMM-s 38^, or 70^ below freezing point. In 1862 

 Mr. Glaisher and Mr. Coxwell made their famous ascent, 

 when they reached an altitude of about seven miles from 

 the earth. A short time ago a balloon, without an 

 aeronaut, but having a set of self-recording instruments 

 attached, was sent up in France, and from the records 

 obtained it is shown that a height of about ten miles was 

 attained, and that the temperature fell to minus 104" Fahr. 



Clouds are simply a form of water made visible by the 

 cooling of the air which previously held the water in the 

 form of invisible vapour. Every cloud may be regarded 

 as the top of an invisible warm column or current 

 thrusting its way into a colder body of air. The com- 

 parative altitude of a cloud may be judged, when there is 

 no time or opportunity to make exact measurements, 

 from its form and outline, its shape or shadow, its 

 apparent size and movement, its perspective effect, and 

 the length of time it remains directly illuminated after 

 sunset. By the last method Mr. Inwards states that some 

 clouds have been estimated to have been at least ten 

 mUes above the surface of the earth. The cloud velocities 

 at high altitudes have been carefully noted at the Blue 

 Hill Observatory, Mass., U.S., and show, practically, that 

 at about five miles in height the velocities are three times 

 as fast in summer and six times in winter as the velocities 

 of the currents on the earth's surface. 



Dr. Hume, in his thesis for the Doctorate of Science, 

 has given geologists a minute and exhaustive account, 

 chemical, mineralogical, and biological, of the zones of 

 the upper Cretaceous in the south of England. With 

 regard to the vexed question of the origin of the chalk, 

 so interesting in its bearing upon the permanence of the 

 great oceanic troughs. Dr. Hume is of opinion that the 

 organic remains point to an ocean which, if not abysmal, 

 was at least far deeper locally than many prominent marine 

 areas. The opinion which M. Cayeux formed from a study 

 of the minute derived minerals in the chalk, that this rock 

 is after aU a shallow- water deposit, is contested by Dr. 

 Hume, and he hints not obscurely at ice and ocean currents 

 to explain the mineralogical difference between the chalk 

 detritus and the corresponding constituent of the globi- 

 gerina ooze now accumulating in the Atlantic. 



INSECT SECRETIONS.-I. 



By E. A. Butler, B.A., B.Sc. 



AS a gi"oup, insects are remarkable, when compared 

 with the other great divisions of the animal 

 kingdom, for the number and variety of the 

 secretions they produce, whether it be for pro- 

 tection against their numerous enemies, or for 

 constructive or other purposes connected with their own 

 economy. Not only are these secretions useful adjimcts 

 to the creatm-es from which they emanate, but they are 

 also the source fi'om which most of the valuable products 

 obtained by man directly from the insect world are derived. 

 Such materials as wax, silk, lac, cochineal, all of which 

 are of the nature of secretions, will at once occur to the 

 mind and serve to exemplify the variety mentioned above, 

 and the commercial advantage that may accrue when man 

 can take under his own control the wonderful secreting 

 power of these little beings. But these four products, 

 though amongst the chief of what we have found it profit- 

 able to employ for our own purposes, and therefore by far 

 the best known are but a very small proportion of the 



multitude of substances secreted by insects ; and we pro- 

 pose in the present series of papers to try to gire some 

 idea of the immense number and variety of these products 

 of insect vitahty. The subject will open up, to such of our 

 readers as possess the requisite qualifications and care to 

 pursue the inquiry, a wide field of investigation, for in many 

 cases but little is known of the exact composition of the 

 substance secreted, and the discoveries of the last few 

 years have shown that in aU probability many more exist 

 than are yet known or even supposed. 



It may be premised that such secreted substances as we 

 are here speaking of will, in their primitive condition, be 

 liquids, though these may afterwards, when stored or 

 brought into actual use, either solidify or vaporize ; and 

 further, that the presence of a secretion implies the 

 existence of a special apparatus for the purpose of 

 elaborating it. To this apparatus the general name of 

 " gland" is applied, whatever may be its exact structure. 

 To constitute a gland, all that is essential is a cell, or a 

 group of cells, which shall have the power, under nervous 

 stimulus, of elaborating from the materials with which 

 they are supplied the secretion in question. There wiU 

 in most cases also be some means by which the secretion, 

 when formed, can be conveyed to the spot where it is 

 required, and this will fi-equently be in the form of a tube 

 or tubes called " ducts." There may also be superadded 

 to these parts a storage reservoir, especially when the 

 secretion, though tolerably constant in its formation, is 

 used only at intervals. Of course such substances as bile, 

 saliva, &c., which occur in many groups in the animal king- 

 dom, are of the nature of secretions, but we do not propose 

 to deal here with such as are common to many classes 

 of animals, but only with those that, with one or two 

 slight exceptions, are characteristic of the class Insecta. 

 These secretions are not confined to any particular gi-oup 

 of insects, and in fact, of all the eight or nine orders into 

 which the majority of insects naturally fall, not more 

 than one or two will be omitted from our enumeration as 

 being of insignificant importance in this direction. By 

 far the most extensive group thus excluded is the order 

 Diptera, or two- winged flies, to which we look in vain for 

 any noteworthy secretions of the kind we are speaking of. 

 Even the formation of silk, which is so common amongst 

 other insects with a complete metamorphosis, is practically 

 absent fi'om this order. One good reason for this wiU 

 readily occur to the mind. One of the chief uses of silk 

 to insects in general is to protect them in their helpless 

 pupal condition ; but a large number of the flies, it will be 

 remembered, form their chrysaHs inside the last larval 

 skin, which hardens into a ban-el-shaped receptacle, the 

 whole structure thus formed going by the name of 

 " puparium." Thus the chrysalis, being ah-eady protected 

 by the hard barrel-like skin of the larva, requires no other 

 envelope, and the silk-forming power is consequently absent. 



In silk we have the chief secretion 

 which is shared by animals outside the 

 limits of the class Insecta ; spiders, 

 in making receptacles for their eggs 

 (Fig. 1) (the so-called cocoons), and in 

 building up their snares, use a gummy 

 secretion drawn out into a fine 

 thread, which is sometimes scarcely 

 distinguishable from the sUk of 

 insects. The sUk of which their egg- 

 cases are formed is most like that 

 of insects, as is evidenced by the 

 apphcation of the term "cocoon" to 

 them, though they shelter eggs and 

 not pupa. The silk of which the 



Fia. 1. — Spider'i 

 oocoon suspended from 

 twig. 



