INSECT VISITORS 



139 



while the 'flasks' occur in various parts on the distal joints. Such 'flasks' are 

 found in Apis, Bombus, Eucera, Xylocopa, and Anthophora. The olfactory pits 

 and olfactory cones vary greatly in number. In honey-bees there are 14,000 to 

 15,000 pits, and some 200 cones on each antenna. 



Among Diptera there are chitinous pits containing sensory cones, and of 

 very varied forms. The pits are sometimes simple, with only one cone ; sometimes 

 compound, with a larger number (up to 100) of cones. The Tipulidae possess 

 isolated cones only, while both kinds are present in Tabanidae, Asilidae, Bomby- 

 liidae, Leptidae, Dolichopodidae, and Stratiomyidae. In the other families there 

 are only aggregated cones. In the flesh-flies and dung-flies there are 60 to 150 

 pits, in Trypeta and others only 2 to 5 on each antenna. 



A few of the Neuroptera, Orthoptera, and Hemiptera also possess pits 

 or sensory cones on the antenna. 



The centre of the olfactory sense of insects is to be sought ai the base of 

 ilie antennary nerve. At its point of origin occur peculiar rounded masses, the 

 olfactory bodies, which are so described. 



As there is a well-developed sense of taste in many insects, it may be deduced 

 that they also possess special gtistatory organs, since in the case of odourless 

 substances taste is only possible when these are touched 

 by the mouth-parts of the insect, its seat must be in the 

 region of the mouth. Sensory pits with nerve-endings 

 are actually found (Kolbe, op. cit., pp. 442-5) both in 

 the walls of the mouth-cavity as well as on the tongue 

 and palps. These must at once come into contact with 

 the food that is taken. On the proboscis of a fly, lor 

 example, organs of taste are found, together with tactile 

 hairs. The interior of the tubular proboscis of a butterfly 

 or moth is regularly beset with small chitinous cylinders, 

 which project into its cavity, undertaking a quantitative 

 and qualitative examination of the fluid nourishment 

 taken in. At the base of the tongue in bees and similar 

 insects there are little chitinous pits on either side, which 



are regarded as gustatory organs. At the tip of the ligula and on the under-side 

 of the first maxillae of some Hymenoptera (ants, wasps, bees) sense-organs (Fig. 56) 

 are also present. The paraglossae of the honey-bee and humble-bees also possess 

 similar taste-organs. 



It is a familiar fact that insects possess compound or /aceied eyes, the peculiar 

 structure of which is explained by their immobility and the slight power of move- 

 ment possessed by the head. If insects were equipped with a single refractive 

 apparatus like the eyes of a vertebrate, but immovable, this would need to be 

 very strongly convex, and very prominent, to enable its owner to see at the same time 

 objects in front and around. But such an arrangement would make distinct vision 

 impossible on account of the well-known principle of spherical aberration. When 

 eye and head are immovable, clear vision can only be attained by division of the 

 eye into a number of radiating cones, capable of making isolated and independent 

 observations, and so arranged that rays of light, from any object within the field 



Fig. ^6. Taste-organs of In- 

 sects. Sensory cone {Sk) from 

 the tip of the ligula of Vespa 

 vulgaris; ..^y; axial thread. (After 

 O. vom Rath.) 



