In making a comparative study of the olfactory pores in beetles, 50 species belonging to 47 genera and representing 34 families were used. A group of pores is always present on the peduncle of each elytron. It lies on the dorsal side of the well-exposed radial plate. The number of pores on a pair of elytra varies from 12 to 310. As a rule, the more pores in the group the smaller they are and the closer they are together. Of the 47 winged species examined, 11 have only one group of pores on each wing, 21 have two groups on each wing, 12 have three groups on each wing, and 3 have four groups on each wing. These groups are always located on the dorsal surface. Only occasionally are a few scattered pores found on the ventral side of a wing. When one or two groups are present, they lie on the radius. When three groups are present, all three may lie on the radius, or two may lie on the radius and the third on the media. When four groups are present, one lies on the subcosta, two on the radius and one on the media. The largest group on the radius usually extends nearly all the way to the fold of the wing and sometimes all the distance to the fold. The number of pores on a pair of wings varies from 130 to 982. There are usually two groups of pores at the proximal end of each trochanter. Sometimes a pore is found at the proximal end of the femur. It is common to find a few pores at the proximal end of each tibia; and sometimes pores are found in the tibial spines and on the tarsi. The number of pores on all six legs varies from 49 to 341. In regard to water beetles, the better the legs are adapted for locomotion in water, the fewer pores they have. The smallest winged species (Coxelus) examined has 273 pores, which is the smallest number of all the species, and the largest species (Orthosoma) has 1,268 pores which is the largest number of all the species examined. The apterous species have more pores on the legs than usual. As a rule, the smaller the species, the fewer its pores and the larger they are, comparatively speaking. As a rule, there are no generic and specific differences, except variations in number of pores, the amount of variation depending on the sizes of the individuals compared. There are no individual and sexual differences other than slight variations in number of pores. The pore apertures or pits are round, oblong, slitlike or club-shaped. On the elytra and wings they are always round or oblong. On the legs they have all four of the enumerated shapes. The spindle-shaped sense cells of most beetles lie in the lumens of the appendages outside the pore cavities, but in the legs of Orthosoma the sense cells lie inside the pore cavities. A small chitinous cone is always present. It is formed by the hypodermal cell at the mouth of the pore after the insect has emerged from the last pupal stage, and at the same time when the chitinous integument is being considerably thickened. The sense cells are fully developed when the insect emerges into the imago stage. The sense fiber pierces the cone and the layer of chitin between the pore aperture and cone, and it enters the bottom of the pore aperture or pit where its peripheral end comes into direct contact with the outside air. In Hymenoptera the sense fibers enter the pore apertures which are almost on a level with the external surface of the chitin. In Coleoptera, with a few exceptions, the sense fibers enter the bottoms of pits which lie in the chitin one third (at time of emerging into imago stage) the distance from the external surface. In the legs of the lady beetle, Epilachna borealis, instead of the chitin which surrounds the pore apertures being depressed, it is elevated so that the pore apertures lie in the center of domes above the general surface of the legs. In the legs of the blister beetles, Epicauta marginata and E. pennsylvanica, the pore apertures lie on a level with the surface of the legs. In the legs of the potato beetle, the pore apertures lie at the bottoms of shallow pits. All four preceding species have hypodermal gland pores over the entire body, except the wings. These pores in the lady beetle are perhaps the most highly developed. They lie on all sides and even among the olfactory pores on the legs. In the other three species they are less highly developed on the legs near the olfactory pores and none is found very close to an olfactory pore. This correlation between the hypodermal gland pores and the olfactory pores is certainly a means of preventing the secretion from the gland cells from running into the pore apertures. A large nerve and a large trachea run into each elytron and wing. In the peduncle of the elytron they run through the radial plate just beneath the group of olfactory pores. Branches from the nerve are given off which connect with the sense cells. The large nerve and trachea passing into the wing soon divide so that a smaller nerve and a smaller trachea run through each main nerve. The largest trachea passes through the subcosta, and the largest nerves pass through the veins carrying the olfactory pores. These nerves give off branches which connect with the sense cells. The sense cells wherever found are always surrounded by blood. In the experiments to determine the location of the olfactory organs, 434 individuals were tested. These belonged to 11 species representing 8 families. After the antennæ were pulled off, 4 of the 11 species tested were normal and 7 were slightly abnormal in behavior. After the elytra and wings were pulled off 1 species was normal while 4 were slightly abnormal in behavior. After the elytra were pulled off and the wings were cut off, the 1 species tested was normal in behavior. After the elytra and wings were pulled off and the pores on the legs were covered with vaseline, the 1 species tested was slightly abnormal in behavior. After the elytra were pulled off, the bases of the wings glued and the pores on the...