Chemoreception 453 



only individual odors but also caste odor, hive odor, sting odor, and wax odor."** 

 The most elemental and important chemical attractants are those which orient 

 insects to foods. These attractants are, for the most part, odors. Often these 

 odors are only sign posts indicating the presence of foods which do not con- 

 tribute to the odor. Thus the beetle, Creophilus, orients to odors of decay- 

 ing meat only to feed on the fly maggots developing there; most caterpillars 

 are oriented to their food plants by the odors of essential oils, yet they de- 

 rive no nutritional benefit from these oils. Once insects are oriented in the 

 vicinity of food, other appropriate stimuli may be necessary to initiate feeding 

 and for the continuation of feeding. Thus, wireworms aggregated in the 

 vicinity of food fail to initiate eating, and larvae of Anosia plexip^is, the milk 

 weed feeder, will orient to strange leaves coated with milk weed latex but 

 will spit out the first mouthful and will starve rather than eat the strange 

 food.i« 



Many insects possess a high degree of sensitivity to water vapor, and their 

 behavior is affected by relative humidity (Ch. 2). In many respects this 

 response to water vapor resembles that to olfactory stimulation. The sense 

 organs necessary for orientation to water vapor are the pegs and pits located 

 on the antennae.-" The acuity of this sense is surprising: the mosquito Cidex 

 fatigans can distinguish differences of 1 per cent relative humidity near 

 saturation, and wireworms can distinguish between 100 and 95.5 per cent 

 relative humidity. The response of organisms to water vapor is not always 

 constant and may vary with the water balance of the animal. 



Other Arthropods. Crustacea possess a definite chemical sense which in 

 decapods seems to be largely localized in the mouth appendages and the an- 

 tennules. The external ramus of the antennules is well supplied with basi- 

 conical hair organs variously known as aesthetascs, olfactory clubs or tubes, 

 and tubules of Leydig. Other parts of the body are also chemoreceptive, 

 even the hard carapace, which is supplied with numerous pores, at the base 

 of which are sensory cells. 



Lhmdiis apparently has contact chemoreceptors in the mandibles and 

 chelae, and distance chemoreceptors in a wartlike bud anterior to the mouth. 



There is a considerable amount of controversial literature concerning the 

 regional distribution of chemoreceptors in spiders. fH^owever, evidence indi- 

 cates that they are widely distributed over the body, especially in the palps, 

 the first two pairs of legs, and possibly in the mouthparts. 



Chemoreception in the Vertebrates. A high degree of structural and func- 

 tional specialization of cell groups to mediate the senses of olfaction, taste, 

 and the common chemical sense has been achieved by many vertebrates. In 

 all vertebrates, with the possible exception of the reptiles, birds, and man, 

 olfaction and taste are probably necessary for survival. The common chemi- 

 cal sense probably serves organisms in a manner analogous to the pain sense, 

 and therefore has survival value. 



STRUCTURE. The morphology of chemoreceptors is much less diversified 

 in vertebrates than in the insects, and the sense organs have been studied in 

 detail in many organisms. The sense cells which mediate olfaction in man 

 are neurones located in the upper part of the nasal ca\ity. The olfactory end- 

 organs cover a small area (2.5 square centimeters for each nostril of man) 



