Luminosity of Midges. 137 



midges vary according- to the individual species even much 

 more than do the imagines, and this favours the beUef that 

 Chironomus plumosus^ L., or Ch. intermedius^ Fb., the form 

 which is so closely allied to it, really occurs at Lake Tssykkul, 



For the rest, however, the agreement of the diagnosis with 

 the insects examined by me is so striking that I have no 

 doubt whatever as to the correctness of ray determination. 



It consequently follows from my investigation that the 

 luminous midges belong to species which are the commonest 

 and widely distributed, as had also once before been found to 

 be the case by Dr. Brischke {vide supra). 



We now have to discuss the explanation of their luminosity. 



From tlie purely biological standpoint luminosity in animals 

 in general can be referred to two causes. 



In the hrst place animals are luminous, which for this 

 purpose are provided with special luminous organs, and in 

 which this function plays one part or another in their life- 

 history, though the precise role may not always be manifest 

 to us — such as, for instance, our own Lampyris^ several exotic 

 insects, luminous crustaceans and fishes living at great 

 depths in the sea, &c. 



In the second place, there are animals which are luminous 

 by means of luminous micro-organisms living upon them or 

 in them, which indeed, as is well known, also occasion the 

 luminosity of decaying substances (meat, fish, wood, and 

 similar bodies). In this instance we have to distinguish two 

 cases : — 



1. The micro-organisms may be harmless to the animal 

 and we must consequently regard them as " room-parasites " 

 (Eaumparasiten), or even as commensals (since they perhaps 

 also benefit the animal by their luminosity, by attracting 

 its prey to it). The bacteria of the medusa Felagia and 

 of the mollusk Fholas^ discovered by R. Dubois f, may serve 

 as a case in point. 



2. The micro-organisms may occur as veritable parasites, 

 injurious to their host. Of this, however, we are at present 

 acquainted with only a single instance, which was brought 

 forward by A. Giard |, namely the luminosity of Talitrus. 



* Reaumur, ' Mem. pour serv. a I'hist. des Insectes,' vol. v. p. 38, tab. v. 

 figs. 1-5. 



t R. Dubois, " Sur le role de la syrabiose chez certains animaux marina 

 lumiueux," Compt. Reud. Acad. Paris, t. cvii. 1888, p. 502. 



X A. Giard, " Sur I'infection phosplioresc. des Talitres et autres Crus- 

 tacees," Comptes Rendus, Sept. 2.3, 1889, p. 503 ; Ann. & Mag. Nat. Hist. 

 (6) iv. pp. 47t)-478 (1889) ; referred to in Centralbl. f. Bact. u. Parasiten- 

 kunde, Ed. vi. 1889, p. 645 (I quote according to Hericoiu-t, " Les microbes 

 lumineux," Revue Scient. t. xlv. 1890, no, 15, p. 465). 



