The Hymenoptera furnished very good material for histological purposes, so that Hauser could not only study the terminal apparatus of the olfactory nerves in the perfect insect, but also in three different stages of the pupa. These are described at length, as regards the distribution of the pits and teeth, in _Vespa crabro_; each joint of the antenna (flagellum) possesses between 1300 and 1400 pits, nearly 60 teeth, and about 70 tactile hairs; on the terminal joint there are more than 200 teeth, so that each antenna has between 13,000 and 14,000 olfactory pits and about 700 teeth (Kegeln). Fig. 278 represents a cross-section through the penultimate antennal joint of _Vespa crabro_; we can see how thick are the series of openings on the surface of the antennæ, and how regular is the distribution of the teeth. The distribution of the olfactory pits and olfactory teeth is thus seen to be very general; the deviations are so insignificant that there is no reason for the establishment of more than one type. Antennal pits with a small crevice-like opening occur in genera nearly allied to Vespa and also in most Ichneumonidæ, Braconidæ, and Cynipidæ. But the crevice-like openings in these families are considerably longer and often are of a somewhat twisted shape. In all the species with translucent antennæ we can recognize the inner mouth of the pit as a round or nearly round disk situated usually under the middle of the opening. The antennal pits of _Apis mellifica_, as well as those of Bombus (Fig. 280) and allied genera, differ from those of the Ichneumonidæ in being not like crevices, but circular openings. [Illustration: FIG. 280.—Olfactory pits of the antenna of Bombus.—After Kraepelin. ] [Illustration: FIG. 281.—Olfactory pits of the antenna of Formica: _Fv_, Hicks’ “bottle,” Forel’s flask-shaped organ, _Fvo_, its opening.—After Kraepelin. ] [Illustration: FIG. 282.—Supposed olfactory organs at end of antenna of Campodea: _A_, _C. staphylinus_. _B_, _C. cookei_, from Mammoth Cave. ] [Illustration: FIG. 283.—Vertical section through a single olfactory pit in the antenna of the horse-fly (_Tabanus bovinus_). For lettering see p. 272.—After Hauser. ]