i HAP. vi. | TROPISMS. 55 



those induced, for example, by the solution of chemicals in the 

 environment of aquatic organisms. The much-lauded " wonderful 

 instinct " which impels a female insect to lay her eggs on the 

 particular plant or substance suitable for the food of the larvae is 

 as a rule merely a mechanical response to definite chemical stimuli 



rted bj tin- appropriate larval food. It has been shown, foi 

 example, that a blowfly will readily lay its eggs or maggots on 

 meat, on which the larvae normally feed, but that it will not lay its 

 eggs on fat, upon which the larvae could not feed. This is not a 

 case of instinct or intelligence but is simply due to the fact that 

 c ertain chemical stimuli from men cause the blowfly to lay its eggs. 

 In I. it these stimuli are lacking but can be produced artilicialh .when 

 tin' fly will lay its eggs although tin 1 larva' cannot \Wi\ on the fat. 



Chemotropism will undoubtedly in the future be put to practical 

 use in economic entomology to a very large extent. Its practical 

 applications .u present include the attraction of moths by assem- 

 bling and by the use of baits such as sugar either painted on trees, 

 etc.. or used in combination with a special trap, and the attraction 

 of some fruit-flies by the use of oils. In these latter cases, tins 

 lining water with a little oil (kerosine, citronella, etc..) are 

 ised in localities in which fruit-tlies occur and the male flies 

 are attracted by the smell of the oil (which doubtless exercises the 

 same tropic reaction as the odour emitted from the scent-glands oi 

 the female of that particular fly) and are drowned in the tins. 



I li< rmotropism. or response to tin- Stimulus oi temperature, is 

 perhaps in some cases scarcely distinct from Chemotropism. 

 Hibernation and /Estivation (the passing oi the cold and hot 

 seasons in an inactive condition) are probably controlled directly 

 b\ thermotropic responses. 



Thigmotropism, or response to the stimulus of contact with solids. 

 isgencralh not a very evident phenomenon. Its positive aspects 

 are strongly exhibited by many spermatozoa, such as those ,,t the 

 cockroach ; if these be examined in a cavity-slip under the micro- 

 scope, they will be found to be adherent onlj to tin- sides of the 

 cavity or the coverglasS and not free in the cavity ; when brought 

 into contact with an egg, they adhere strongly to its surface over 

 which they wander until one reaches and enters the micropyle, thus 

 securing fertilisation. Some insects which are found beneath bark 

 are positively thigmotropic. 



Geotropism, or response to the stimulus of gravity, is exhibited 

 by many caterpillars both positively and negatively at different 

 times. Whilst feeding they are usually negatively geotropii , 

 crawling upwards on anj vertical object should they fall off the 

 food-plant, but when full-fed and about to pupate in many 



