AUTHOR'S .ABSTRACT OF THIS PAPER I~SUED BY THE BIBLIOGRAPHIC UERVICE, JULY 19 THE OLFACTORY ORGANS OF DIPTERA N. E. McINDOO Bureau 0.f Entomology, Washington, D. C. FIFTY-FIVE FIGURES CONTENTS Introduction and methods. ................................................ 457 The olfactory pores.. ......... ...... 460 Disposition of pores in Musca domestica.. ............................. 460 a. Pores on legs ..................................................... 460 b. Pores on wings ................................................... 460 c. Pores on halteres ................................................. 461 Disposition of pores in other species a. Pores on legs.. .................. ................... c. Pores on halteres.. ............................................... 464 d. Pores on abnormal species.. ' . e. Generic, specific, individual, an Structure of pores in Musca domestica................................. 471 a. External structure.. .. ........... .............471 b. Internal structure.. ... Structure of pores in other ................ 478 a. External structure.. .............................................. 478 b. Internal structure ................................................ 479 The antennal organ ............................. Summary. ......... ............................. I,itcrat,ure cited. ............................ ........................ 484 INTRODUCTION AND h'IETIIODS The results herein recorded are a continuation of the writer's investigation concerning the morphology of the olfactory pores. Up to date, including the present results, these organs have been carefully studied in Hymenoptera, Coleoptera, Lepidoptera, and Diptera. The chief object of the present investigation is to determine whether the olfactory pores are better adapted ana. tomically than the antennal organs to receive olfactory stirnuli. 457 THE JOURNAL OF COWARATIVE NEUROLOGY, VOL. %, NO. 5 OCPOBER, 1915 458 N. E. McINDOO The investigators who have performed experiments on flies with mutilated antennae have concluded that these appendages bear the olfactory organs, regardless of whether or not the anten- nal organs are anatomically fitted to receive olfactory stimuli. Since these investigators failed to study sufficiently the behavior of the insects investigated, it is possible that the responses observed misled them in determining the seat of the olfactory organs. In 1857 Hicks discovered porelike organs on the wings and halteres of fiies, and claims that they are similar in structure and probably have the same function, that of smell. He was able to trace a nerve to each group of organs, the one going to the halter bking the larger. The same author ('59) found these organs in Hippobosca equina and Tipula olerocea, and in 1860 discovered them on the legs of various insects, including Diptera. In the same year Leydig described and figured the same organs on the halteres of Calliphora (Musca) vomitoria and Eristalis tenax. Each one of the foregoing authors was able to trace nerves to these pores, but they could not understand the internal anat- omy of them. Graber ('82) described and figured these organs on the wings and halteres of several Diptera, and called them chordotonal organs, because he thought the peripheral ends of the sense cells were sensory chords. Lee ('85) described and figured in detail these structures on the halteres of Calliphora vomitoria, but he, like the preceding authors, failed to understand their internal anatomy. The paper of Weinland ('90) is the most comprehensive one dealing with the sense organs found on the halteres, and as a whole it is the best, although he did not clearly understand the anatomy of these structures. He gives a good review of the literature pertaining to the halteres, and according to him the earliest writers (beginning in 1711) said that these appendages served in maintaining the equilibrium of the insect while flying ; hence the Latin name, halteres and the English translation, balanciers. About a century later experiments proved that flies with amputated halteres could fly, although not as well, THE OLFACTORY ORGANS OF DIPTERA 459 and konsequently the preceding view has long since been aban- doned. Another old view was that the halteres aid in respiration Hicks and Lee regarded the structures as olfactory organs, while Leydig and Graber thought they were auditory in function. Weinland determined that the halteres in vibrating rapidly perform a number of different movements, and chiefly for this reason he thinks that the organs borne by them bring about the perception of movements, thereby steering the flight of the insect. He asserts that since the antennae bear the olfactory organs, the organs on the halteres certainly do not perform the same function. Nagel (’94), in commenting on the probable function of the halteres, thinks that the first four preceding views have been abandoned, but, he is a strong advocate of Weinland’s view. The paper of Prashad (’16) seems to be the most recent one concerning the sense organs on the halteres, and this author studied only the halteres of the mosquito, Ochlerotatus pseudo- taeniatus Giles. He evidently did not have access to most of the literature on this subject and consequently has added little knowledge concerning these organs. He thinks that each organ has an external opening and found two scalpel groups of pores on each halter, while the present writer found only one scalpel group on each halter of mosquitoes belonging to other genera. McEwen (’18) has just recently observed the sense organs on the wings of Drosophila ampelophila. He determined “that these organs had nothing to do with the response to light” (pp. 85 to 87), but performed no experiments using odor stimuli. To obtain material for the study of the disposition of the olfactory pores, dried museum specimens were largely used. These specimens were obtained of Messrs. C. T. Greene and C. H. Popenoe through the courtesy of Dr. L. 0. Howard. Mr. Greene is furthermore to be thanked for verifying the identifi- cation of all the species used. Fresh material was fixed in the modified Carnoy’s fluid, and was embedded in celloidin and paraffin. The sections were cut three and five microns in thick- ness, and were stained in Ehrlich’s hematoxylin and eosin. All the drawings were made by the writer and all are original except figures 50 to 55; these represent the antenna1 organs of flies 460 N. E. McINDOO and mosquitoes, and were copied from Hauser, vom Rath, and Nagel. The drawings were made at the base of the microscope with the aid of a camera lucida. THE OLFACTORY PORES Before making a study of the anatomy of the organs, called the olfactory pores by the writer ('14 a), the dist,ribution and number of them were first investigated. Disposition of pores in Musca domesticn Owing to an abundance of material and to thc economic im- portance of the house fly, the olfactory pores of this insect have been studied and drawn in detail, and it is hoped that such work will encourage experimentation along practical lines. a. Pores on legs. Seven goups of pores lie on each leg and the disposition of them is as follows: nos. 1 to 4 on the inner surface of the leg (fig. 1) and nos. 5 to 7 on the outer surface; nos. 1,2, and 5 being on the trochanter, nos. 3 and 6 on the femur, and nos. 4 and 7 on the tibia. Nos. 1 and 2, consisting of 5 and 8 pores, respectively, always lie on the anterior margin of the leg, while no. 3, composed of 11 pores, lies on the posterior margin. Nos. 4 and 7, when present, may lie on eit8heror both margins of tthe leg and the number of pores in each group varies from one to three. No. 5, consisting of 3 pores, usually lies near the poste- rior margin, while no. 6, composed of 1 pore, lies near the anterior margin. b. Pores on wings. Six groups and several scattered pores lie on each wing and the disposition of thcm is as follows: Nos. 8 to 11 and scattered pores a to c lie on the doma1 surface of the wing (fig. 2), while nos. 12 and 13 and the scattered pores d and e lie on the ventral surface. No. 8, consisting of about 24 pores, lies at the proximal end of the propteiygiurn (Pr),while nos. 9 to 13 lie on the subcostal (Sc)vein in about the positions as indicated by the numbers in figure 2. The number of pores in each of these groups varies slightly, but the average number in each is about as follows: no. 9 has 50 pores; no. 10, 12 pores; no. 11, 10 pores: THE OLFACTORY ORGANS OF DIPTERA 461 No. 12, 9 pores, and no. 13, 18 pores. The scattered pores vary considerably in number and position and they are located about as follows: 1 at a on the base of the humeral vein; 2 always present at b on the distal end of the first radial vein; 1 at c on the radiomedial vein; 1 at d on the proximal end of the first radial vein; and 1 at e on the fourth radial vein. Fig. 1 Portions of legs of house fly (Musca dornestica c?), showing location of groups nos. 1 to 7 of o1f:xtory pores. The drawings at the right represent the inner surface and those at the left the outer surfnce. , AntM and PostM stand for anterior and posterior margins. X 20. c. Pores on halteres. Five groups and 1 isolated pore lie on the base of each halter (fig. 3); nos. 14 to 16 and trhe isolated pore at f being found on the dorsal surface and nos. 17 and 18 on the ventral surface. The pores lie on plates whose outlines are simi- lar in shape to the contours of t,he groups of pores themselves; 462 N. E. McINDOO hence, the pores in nos. 14 and 18 have been called scalpel organs because each group lies on a plate shaped like a scalpel. No. 15 lies on the basal plate, consequently its pores have been called basal organs. No. 16 lies 011 the anterior end of the basal plate, while no.