44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



I. PHOTORECEPTORS 



According to the phototactic responses of the Mexican bean beetle 

 and its larva, already discused, the compound eyes and ocelli in this 

 species are normally developed and seem to function adequately, so 

 far as beetles are concerned. It is recalled that the adults are always 

 photopositive and that the larvae up to the time of pupation are photo- 

 positive, too, but w^hen ready to pupate they become photonegative. 

 Whether the negative reaction is caused by a change in the structure 

 of the ocelli is not known. 



Since the morphology of insects eyes has often been discussed and 

 as the writer (46, 47) has recently cited reviews on this subject, no 

 further discussion is needed here. Also, the other sense organs and 

 senses of beetles will be discussed only briefly. 



II. CHEMORECEPTORS 



Chemoreceptors include both olfactory and gustatory organs, but 

 we are not absolutely sure that insects have true chemoreceptors, al- 

 though their organs certainly belong to the same category. 



I. SO-CALLED OLFACTORY ORGANS 

 (a) ANTENNAL ORGANS 



The organs on the antennae of the Mexican bean beetle are com- 

 paratively few ; that is, these antennae are nearly bare in comparison 

 to most antennae (fig. 6). Only four types of sense organs were 

 found on them. They are as follows: (i) Two groups of tiny hairs 

 (St) ; and (2) three or four pores (P), called olfactory by the writer, 

 lie on the base of the first antennal segment; (3) the Johnston organ 

 (/) lies at the distal end of the second segment; and (4) five areas 

 of thin-walled hairs (OHr) were found on the distal ends of the ninth, 

 tenth, and eleventh segments. All of these structures are sense organs, 

 because sense cells were found connected with them, while the larger 

 hairs (Hr), usually called sense bristles, were found to be non- 

 innervated. 



Of these four types of sense organs only the olfactory ix)res and 

 thin-walled hairs may be regarded as so-called olfactory organs. The 

 thin-walled hairs are numerous and most of them He on the dorsal 

 surface of the antennae (fig. 6, OHr). Under a high-power lens they 

 appear long and slender, have thin, almost transparent walls (C, OHr), 

 and are connected with sense cells. 



From the preceding it is evident that pore plates, found only on 

 the antennae of aphids, bees, wasps, and on some beetles, are totally 

 absent on the antennae of the Mexican bean beetle. The pore plates. 



