22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



nent objects or squeeze themselves between layers of folded dry 

 goods. On these facts is based the employment of bands around 

 fruit trees to catch codling-moth larvae and the caterpillars of the 

 Tortricidse of the vine ; likewise the use of stones and boards in 

 gardens for collecting beneath them earwigs and slugs, which can 

 then be destroyed wholesale. Female grape-moths {Clysia am- 

 biguella) , when laying eggs, are guided by the highly sensitive extrem- 

 ity of the abdomen, and this sensitive part aids the females of many 

 insects to lay eggs in fissures and folds of plants, soil, and elsewhere. 

 Dewitz also believes that thigmotaxis is the chief means by which 

 caterpillars and certain other insects are able to live gregarious lives. 

 McCracken (47), while testing female silkworm moths, determined 

 that eggs might be obtained by touching the sense hairs on the ovi- 

 positor with a pencil or fibers of cotton. She says that each contact 

 brings forth an egg, and under natural conditions the stimulus is 

 brought about by means of the ovipositor coming in contact with the 

 surface upon which the insect rests. 



2. TIIIGMOTACTIC EXPERIMENTS ON CODLING-MOTH LARVAE 



All instars of the codling moth seem to have a well-developed sense 

 of touch, but the thigmotactic sense is most pronounced in the fully 

 developed larvae at cocooning time. When ready to spin, these larvae 

 prefer a tight and dark place in which to crawl, but if a dark one is not 

 at hand they do not hesitate to spin in a well-lighted place. Years ago 

 economic entomologists took advantage of this habit by placing 

 " codling-moth sticks " in the rearing jars. The larvae readily spin in 

 these sticks, which later may be transferred to other containers. 



Another practical application of the thigmopositive and photo- 

 negative reactions of these larvae has been utilized for many years. 

 When bands are placed around the trunks of apple trees to serve as a 

 supplementary control method we are merely taking advantage of 

 nature's laws. It therefore seems that so far as tropic responses are 

 concerned the vulnerable period in the life history of codling-moth 

 larvae is brought about by a change in tropisms. 



B. Tropic Receptors 



In discussions of tropisms, sensory receptors are usually implied 

 as being the organs which receive the tropic stimuli, but in plants and 

 the lowest invertebrates specific sense organs apparently do not exist. 

 However, in the higher invertebrates and vertebrates specific sense 

 organs do exist, but with regard to certain sense organs in insects 

 we are still guessing at their functions and consequently cannot 

 accurately connect them with any known tropism. 



