56 v, iME S( 'i i li INDIAN INS] I 1 H VP. VI. 



caterpillars the geotropic reaction becomes positive and they crawl 

 down from the tree or plant to enter the soil. Ladybirds offer a 

 familiar example of a negatively geotropic tendencj as the) always 



tend to crawl upwards and in a natural State this is of practical use 

 to them because the) crawl up to the tips of the twigs of plants 

 and, if Aphids are present, they stop to feed but, if there are no 

 Aphids. they fly off to search another twig. 



Rheotropism, or response to the stimulus of water-currents, is 

 exhibited by many aquatic inserts and other animals such as fish. 

 On streams in the hills various Gerrids ma) usually be seen 

 maintaining their position against the current and on the look-out 

 for any small insects which may have fallen into the water. 



Hydrotropism is exhibited strongly by many aquatic insects; if 

 these are caught and liberated, even many yards awaj from the 

 edge oi a pond, they make straight for the water without any 

 hesitation. The preference tor a dry or a damp habitat is very 

 distinctl) shown by many insects and other animals and may 

 sometimes be turned to practical account, as by the Hooding of 

 fields to dislodge crickets, etc. 



A.nemotropism, or the response to the stimulus of air current--, 

 is closel) akin to rheotropism and in both cases it is usually found 

 that insects tend to orient themselves against the prevalent current. 

 llie large migration-flights of butterflies just before the break of 

 the monsoon are familiar sights in many localities, the air being 

 sometimes thick with thousands of butterflies all hurrying impetu- 

 ously in one direction. The fact that these migration-flights 

 generally take place in India about May and November seems to 

 point to the possibility of their being due to anemotropic tendem ies 

 on the part of the butterflies concerned. According to my own 

 observations these flights are almost always against the wind. 



Having considered these various tropic reactions it may 

 be well to add a few words regarding their practical important < ■ 

 In the first place it must be clearly understood that these 

 tropisms are purel) mechanical reactions to physical or chemical 

 stimuli and that these reactions are set up and must necessarily 

 and involuntarily be carried out by the organism concerned 

 regardless "I its own benefll or injury thereby. A moth immolates 

 itself in a lighted lamp, not because it likes the light nor on account 

 of curiosity; the light probably has a definite chemical reaction on 

 its muscles so that its head is perforce drawn (by the contraction of 

 the muscles) until it is oriented towards the light aftei which its 

 movements in Hying can only lead it to the source of light. Thus 

 the rapidly-moving insect gets into the flame before the heat has 



time to check it in its (light. If one eye is covered with an Opaque 



