on the vision of Arthropods. 399 



influence the ocelli have on the locomotive faculties, or 

 even more limitedly, what influence they have on the 

 guiding of flight. It is not therefore a matter of 

 surprise that the function of the ocelH in insects pos- 

 sessing also compound eyes has not yet been deciphered. 



There are also some serious objections arising from 

 the imperfections of the methods used. It is admitted 

 by the experimenter that the black oil is not quite 

 effective (" que les insectes dont les yeux sont enduits 

 percoivent encore unpen de lumiere " (pt. 3, p. 29) ), and 

 it is evident that this perception may possibly have an 

 important influence on their movements. The method 

 of incising the nerves is open to still more serious 

 objections, for, according to Plateau, it usually kills the 

 insect, or so stuns it that satisfactory observations are 

 difficult to make. It is true that he states that large 

 Diptera bear the mutilation better than other insects, 

 but when he comes to detail his observations on them it 

 is evident from his remarks that this method is not 

 trustworthy. When three specimens of Eristalis (pt. 3, 

 p. 31), on being released, after having the nerves of 

 their ocelli and compound eyes incised, refused to fly and 

 fell to the ground, he says it may be because they had 

 been too violently treated. On the other hand, when 

 incised specimens flew away in an upward direction, 

 there is, of course, no sufficient evidence that the incision 

 had been carried out in a quite perfect manner : in such 

 a case the insect should have been recaptured, killed, and 

 submitted to a careful post-mortem examination in order 

 to ascertain whether all the four incisions made had 

 completely severed the nerves. 



Taking all these points into consideration, I must look 

 on this part of Plateau's observations as of minor value, 

 and shall therefore not give the details of his experi- 

 ments, but merely translate his summary of their 

 results.— D. S.] 



From the attempts of the earlier naturalists, Hooke, 

 Swammerdam, Eeaumur, Marcel de Serres, Duges ; from 

 the researches of Forel, and from my own numerous 

 experiments, two sets of results may be deduced ; the 

 first are facts, not lightly to be contested ; the others are 

 hypotheses of a plausible nature. The results in the 

 first category are : — 



1. Winged diurnal insects — Hymenoptera, Diptera, 



