452 Comparative Animal Physiology 



sess a covering of cuticle which is partially or completely thin-walled; they 

 are innervated by a group of bipolar sense cells; and each possesses minute 

 refrigerant bodies, situated on the distal process of the neurones (Fig. 145). 

 The olfactory chemoreceptors are commonly found on the antennae, but they 

 also occur on the maxillary and labial palps or their homologues. The recep- 

 tors on the antennae are generally conceded to be most sensitive, whereas 

 those on the palps are used for the detection of strong odors.-*' The morpho- 

 logical nature of the chemoreceptors which normally function in feeding and 

 oviposition among the insects is uncertain, and the pertinent data are reviewed 

 by Dethier and Chadwick.-** Data advanced by Frings and O'Neal''" indi- 

 cate that chemoreception in the labella and the legs of the horsefly, Tahanus, 

 is mediated by the medium-sized thin-walled hairs or by undiscovered sense 

 organs in the integument beneath these hairs. Dethier^^ has advanced evi- 

 dence that long and short thin-walled spines located on the surface of the 

 pre-oral cavity of lepidopteran larvae may function in contact chemoreception. 

 Sense organs functioning in contact chemoreception have been demonstrated 

 on the antennae of bees and ants, the mouthparts or adjacent surfaces of 

 many species, the distal segments of the legs of bees, flies, and adult Lepi- 

 doptera, and the ovipositor of ichneumonids and braconids, and of Gryllus.-^ 

 Regarding the sense organs which mediate the common chemical sense, nei- 

 ther their structure nor their distribution is known. 



BEHAVIOR. The role played by the chemoreceptors in the behavior of in- 

 sects is in itself a fascinating story, which is here only touched upon. Natu- 

 rally occurring odors assist certain insects in recognizing and locating their 

 mates, in finding the proper oviposition site, in recognizing their fellows, and 

 in finding food. The fabulous story of the ability of certain moths (e.g., the 

 gypsy moth and others) to attract the males of the species from distances 

 greater than two miles by virtue of the liberation of an attractive odor from 

 the scent organs is widely known. That it is an odor which attracts the 

 males is attested by the fact that males deprived of their antennae do not 

 orient to freshly pupated females, and by the fact that males, in the absence 

 of the female, will attempt copulation with the excised scent gland, with a 

 piece of blotting paper previously touched to the scent gland, or even with a 

 male which recently copulated and retained some of the female sex scent. 

 That these odors can orient males over siich great distances staggers the 

 imagination. Methyl eugenol is used to collect male oriental fruit flies 

 from at least a quarter mile radius. The effect of odors in attracting gravid 

 females to optimal oviposition sites has been shown for the parasitic 

 Mymcnoptcra, a variety of Diptera, and the phytophagous insects. Not only 

 do odors attract the females but often they actually induce oviposition. The 

 parasitic Hymenoptera are particularly outstanding in the ease with which 

 they locate larval hosts buried sev. ral centimeters in wood. The female phy- 

 tophagous insects in general are oriented to lay their eggs on those plants which 

 will serve as food for their larvae. The role of odors in the recognition of nest 

 mates has been demonstrated. Ants which have been washed with water or 

 smeared with body juices of other ants are treated as intruders, and ants which 

 have been deprived of their antennae show little tendency to fight ants of other 

 genera. Social insects like bees live in a world of odors; they distinguish not 



