November 1, 1894.] 



KNOWLEDGE 



245 



'' As to the manner in which the bacteria produce the 

 light, there is still much research needed. As Hulme 

 found in 1800 (and his observation has since been repeatedly 

 confirmed) putrefaction does not assist phosphorescence. 

 The light-producing bacteria are unable to do their work 

 in a substance on which the putrefactive organisms are 

 growing, and as soon as decay ia fairly advanced the light 

 altogether ceases. 



The presence of oxygen appears to be an essential, for 

 colonies wiU only give light on the surface of the culture 

 medium, where they can have free contact with the 

 atmospheric oxygen. This gas, however, is not essential 

 for the hfe of the bacteria. They will grow in an 

 atmosphere of hydrogen or carbonic acid gas, but under 

 such conditions will not produce light. 



Apparently it is not necessary for the colonies to be 

 grown in the light of the sun, for cultivations made in 

 complete darkness have been found to emit light as readily 

 as those grown in daylight. 



It has been suggested by Lehmann and Tollhausen that 

 the light is produced by some molecular change within 

 the cells of the bacteria — a sort of vital process — and the 

 fact that all chemical reagents which destroy the proto- 

 plasm of the cell simultaneously stop the luminosity, lends 

 some support to this theory. Thus, mineral acids, alkalies, 

 and various disinfectants which are fatal to the bacteria, 

 effectually destroy the phosphorescence produced by them. 

 As was mentioned before, heat and cold have a similar 

 effect. 



An equally plausible theory, however, is that just as 

 some bacteria produce ptomaines and albumoses, and 

 others colouring matter, so the light-producing bacteria 

 may produce substances with their molecules so arranged 

 that we have the phenomenon of phosphorescence. If 

 this be so, it is probable also that these products are 

 readily affected by heat and chemical agents, and that then 

 a fresh arrangement of the molecules takes place, and the 

 substances lose their luminous properties. The experiments 

 of Dubois '• on the luminous mollusc Pholas dactylus tend 

 by analogy to support this. From its luminous mantle he 

 extracted two phosphorescent crystalline substances, to 

 one of which he gave the name of huifcrase. It was to 

 these substances, which it secreted, he considered that the 

 fish owed its luminosity. 



When it has been decided whether the bacteria are in 

 themselves phosphorescent, or whether they are so only 

 by virtue of their products, there will still remain the 

 further problem of the nature of the phosphorescence itself. 



THE DADDY-LONGLEGS. 



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



THE advancing autumn has brought with it the usual 

 visitation of swarms of "daddy-longlegs," and it 

 will no doubt be acceptable to the readers of 

 Knowledge if we take the opportunity, while the 

 discomforts of the visitation are still fresh in the 

 mind, of setting before them an account of these fragile 

 but none the less troublesome insects. The chief cause 

 of the inconvenience to which they subject us in their 

 adult state is the awkward way in which they tumble 

 about, blundering up against us with the tickling sensation 

 of buzzing wings and straggling legs, or immolating them- 

 selves in the gas or lamp flame, and startlingly dropping 

 their singed and mutilated bodies on to the page which we 



* Soc. de Bio., Comptes rendus, 1887. 



may happen to be reading or writing. But, as we have 

 had occasion to remark before, it often happens that the 

 most harmful period of an insect's life is not that which 

 is most prominently before human eyes ; the greatest 

 damage wrought by an insect pest is often done in secret, 

 when the real cause of the injuries is generally unsuspected. 

 Such is the case with the insect now before us, for the 

 inconvenience caused during its period of publicity is as 

 nothing compared with the havoc wrought by it during its 

 earlier life of seclusion, when its aspect is so different 

 from that of the well-known " daddy " that none who 

 were not in the secret would suspect the identity of the 

 two insects. 



The "daddy-longlegs" with which we are most familiar 

 is but one species of a large group, the family Tipulida, 

 and of one very extensive genus in that family, the typical 

 genus Tipula. Thus the name " daddy-longlegs " is, 

 strictly speaking, not a specific designation, but a general 

 term, and there are large numbers of insects to which it 

 may be, and is, equally appropriately applied. Still, it is 

 no doubt a single species which is usually understood by 

 the term — a greyish-brown fly, with semi-transparent wings, 

 the brown nervures of which stand out distinctly on the 

 lighter background. The thorax is hoary beneath, and 

 the six long legs are brown at the base and blackish 

 towards the tip. To this species the name Tipulu oleracea 

 has been given. The great length and slenderness of legs 

 in these creatures has recalled the corresponding feature 

 in wading birds, and has led to their getting the name of 

 "crane-flies." In France they are known as "tailors" 

 and " seamstresses." The great length of legs is not 

 altogether disproportionate ; it finds correlated characters 

 in the other parts of the body, and is no doubt of some 

 assistance to the insects in walking in the grassy places 

 that form their principal habitat. 



We may now endeavour to get such an exact notion of 

 the form and structure of our crane-fly as will be obtained 

 by a close examination, assisted by the use of a hand-lens 

 of low power. The single pair of wings marks it out as a 

 dipterous insect, and we may at once notice that the wings 

 are usually carried, when at rest, not folded together oyer 

 the body, as would be the case with most flies, but widely 

 open and slightly elevated on each side, as though to be 

 ready for use at a moment's notice. Their extreme narrow- 

 ness at the base, as well as for some distance along their 

 length, is indicative of that feebleness of flight for which 

 the insects are noted — a feebleness which is, however, 

 perfectly compatible with a rapid and rattling vibration of 

 the wings. 



If this wing be compared with that of a strong flier, 

 such as a bluebottle, a striking difference is seen. The 

 bluebottle's wing is furnished with a sort of extra flap of 

 membrane at its base, which, when the wing is extended, 

 fills up the space between its broader part and the body ; 

 while in the crane-fly this space is quite open and un- 

 occupied with membrane. This appendage to the true 

 wing is called the " alula " or winglet. Projecting from 

 the hinder part of the thorax into this open space is, on 

 each side, a deUoate little organ, the so-called "balancer," 

 a sort of clubbed stalk, whose intimate structure is well 

 worth careful study. Now it is a curious fact that in the 

 two flies we have mentioned, these two parts, the winglets 

 and balancers, appear in inverse ratio of size. The strong 

 and vigorous bluebottle, which has a very large winglet of 

 most exquisite structure, has a completely insignificant 

 balancer, which requires close search to discover it at all ; 

 whereas the weakling crane-fly has no trace of a winglet, 

 but has proportionately the largest balancer that is to be 

 found amongst British Diptera. And this is only one 



