1912] Ecology of Dragon-fly Nymphs 281 
INHIBITION OF PHOTIC RESPONSE. 
It is evident from the above observations that the reactions 
to light may be largely overcome by the response to contact. 
The efforts of the nymphs to swim away from the light are 
inhibited by thigmotaxis. The thigmotactic tendencies are 
evidenced by the organisms forming rather closely grouped 
aggregations. Various authors have called attention to some- 
what similar facts. Formerly, comparatively little importance 
seems to have been attached to such observations, at least, 
among insects. More recently, however, certain workers 
have been impressed with phenomena of this nature. Holmes 
(1905, pp. 324-325) in his experiments with Ranatra has ob- 
served that, “‘The phototactic responses of Ranatra which 
usually occur with such regularity and precision are sometimes 
checked when the insect is engaged in performing some other 
finchiOnee - -  o. Vhtoris to,gortoward the light) are tre- 
quently inhibited by contact stimuli. When several individ- 
uals are put into a dish of water near a window they commonly 
cease, after a time, to swim towards the light and form a cluster 
in which they lie at all possible angles to the direction of the 
rays. The same writer (1905, p. 320) also describes how 
Ranatras group themselves into ‘‘a dense bunch at the negative 
end ’’of the dish. Again, Holmes (1905, p. 323) while working 
with the same aquatic forms, states that, ‘‘In cool water there 
is a marked tendency to form a dense cluster in the negative 
end of the dish.’’ Severin and Severin (1911, pp. 100-101) 
in connection with some work on the thigmotactic responses of 
Belostoma flumineum Say make the following statement: 
‘“‘Again, it was not unusual to find two or more Belostomas or 
somewhat larger clusters clinging together at the surface or 
bottom of the water, a characteristic which is also noticed with 
Lethocerus (=Belostoma Aucct.) americanum, Benacus griseus, 
Nepa apiculata, Ranatra americana, and Ranatra kirkaldyi. This 
habit is probably a manisfestation of their thigmotactic re- 
sponse.’’ This tendency to cluster together has frequently 
been observed by the writer in the case of Gerris remigis Say. 
CHANGES OF BODILY CONDITIONS. 
Much very valuable work has been done upon changes of 
bodily condition, especially among the Protozoa, as for example 
the researches of Putter (1900), Moore (1903), and particularly 
