E. Rivnay.
The Tropisms Effecting Copulation in the Bed Bug.
Psyche 40(4):115-120, 1933.

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19331 Tropisms Effecting Copulation in the Bed Bug 115 THE TROPISMS EFFECTING COPULATION IN THE BED BUG.
BY EZEKIEL RIVNAY
The fact that the female of the bed bug has a special asymmetrical opening for the reception of the male copulatory organs, rather than the ordinary vagina, makes this study extremely interesting. It is astonishing that this fact escaped the attention of early investigators and that the copulatory organs of so well-known an insect were unknown until comparatively recent years. A complete and detailed description of copulation in bed bugs was pub- Fig. 1 Fig. 2
Ventral view of abdomen of the bed bug; Fig. 1, male; Fig. 2, female lished by Hase (1918), and the most complete study of the sexual organs and their functions was made by Cragg (1914, 1920 and 1925). Copulation of bed bugs was ob- served by the present writer during the course of rearing them in connection with other studies. The peculiarity in this act is that the male mounts the female obliquely in such a manner that his head falls over the left side of the pronotum of the female; his left legs grasp the posterior part of her abdominal margin; and his posterior abdominal segment is bent deeply so that the tip reaches the right side



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116 Psyche [ December
of the ventral segments of the female where the copulatory slit is located (Fig. 2). They remain in this position any- where from one to several minutes.
No attempt was made by previous observers to establish the facts as to how the male locates the female, and what tropic reactions govern the act of copulation, which is so unique in this insect. In the following paragraphs, the re- sults of a study are given wherein the writer attempts to discover some of the facts regarding this problem. In some insects, chemotropic reactions are of extreme importance in bringing together the two sexes of the same species for mating purposes. Mayer (1900) demonstrated what attracting force the odor from the female of the C. prometha moth has upon the male. In the case of the Jap- anese beetle, the female apparently diffuses some odor which is attractive to the males. On several occasions, the writer observed in the field, early in the season, over twenty males clustered around one female. Several of these males were attempting to mate with each other, a phase which will be discussed later. The ball-formed cluster, with the female in the center, would lead one to believe that some odor emanating from her attracted all these males. However, observations and experiments described below indicate that the "smell mind" idea cannot be applied in the case of Cimex lectula~w.
A female approached a male from the rear and remained in this vicinity for a long time, yet the male did not per- ceive her. Only when the female passed in front of the male did the latter take cognizance of her and soon made at- tempts to mate with her. Such observations occurred upon other occasions. If odor signals the presence of a female, its effects should be manifested regardless of the position of the female, as long as she is in the proximity of the male. This did not seem to be the case in the above-mentioned observations. In order to verify these casual observations, the following experiments were conducted. Five females were placed in a vial, 2 cm. in diameter and a piece of cheese-cloth was spread and fastened over its opening. This was then turned upside down so that the cloth served as the foothold for the five bugs; it was



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1933 I Tropisms Effecting Copulation in the Bed Bug 117 then placed in a petri dish, allowing space for free circula- tion of air and odor. A similar vial, not containing bugs, was placed in the dish as a control. Five vigorous males were put in the dish and left there for 30 minutes. Al- though the males were in close proximity of the females, they did not pay any attention to them but contrarily made attempts to mate with each other. This experiment was repeated several times with similar results. If there is a secretion, the odor of which serves to entice the male, one would expect this odor to be characteristic of the female only. As a result, one would presuppose that a few males, if brought near females, would cluster around them or around their container in the attempt to mate with them. This was not the case in the experiments. Again, three males were placed in a petri dish. Not long after, one female was placed together with them. All re- mained inactive because the room temperature was rather low. Upon increasing the temperature slightly, the bugs became more active. One male soon mated with the female, while the other male attempted to copulate with the third one. Contrary to expectation, the three males did not crowd around the single female as would have been the case had the female radiated some attractive odor. From the observations mentioned previously, the writer was inclined to believe that it is more likely that the sight of the moving image guides the male to the female rather than the odor. The following experiments were carried out to discover whether such was the case.
A dead female was placed at the rear of a male and was left there for two or three minutes. The male did not ob- serve her. The dead female was then placed about 15 mm. in front of the male and left there for three or four minutes. Here, also, the male did not notice her. The female was then moved slowly with the aid of a fine camel's hair brush, imitating the crawling of a live bug. The male, which had remained inert all this time "woke up," extended his an- tennae in the direction of the dead female, and suddenly jumped upon her and made every effort to mate with her. Similar procedures were carried out several times and also at other occasions with different males, and very often sim-



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118 Psyche [December
ilar results were obtained. On one occasion, the two were tipped over so that the writer could observe, under the microscope, the protruded genitalia, attempting to open the copulatory slit situated between the fourth and fifth ab- dominal sternites of the dead female.
The question will arise as to whether the dead female bed bug had some odor emanating from her which stimulated the male. It might, also, be suggested that the odor of the glandular secretion, if there is such, may be preserved in the female a long time after she is dead. Attempting to clarify this matter, one female was chosen which had been preserved in 70% alcohol for sixteen weeks. After she had been blotted and dried, she was placed in front of the male and was then moved slowly with the brush, imitating the crawling of the live bug. The male reacted to this dead female in the same manner as described heretofore. How- ever, a still better proof that the sight of the crawling female, rather than the odor emanating from her, brings the male to her, may be derived from the following experi- ment.
A piece of cork was carved out into the size and approxi- mate shape of a bed bug. After it was painted with ink and dried, it was placed in front of a male and moved slowly with the brush, again imitating the crawling of a bug. The male suddenly jumped upon it in the same manner as if it were a live female bed bug, but soon dismounted. A few other males reacted in like manner.
It is clear, therefore, that not the odor, but the sight of a moving female, awakens in the male when he is under proper physiologic conditions, the desire to come in direct contact with her. It is also a noteworthy fact that the males lack the faculty of distinguishing between the two sexes, or even a lifeless object of the same size and shape. Recognition of the female occurs through the sense of touch when the male is in direct contact with her. It is then that the male orients himself in a different manner with refer- ence to the surface of the body of the female. Such reactions were observed by the writer in some beetles. In the potato beetle, L. 10-Uneaten, the male is of the same hemispherical shape as the female ; consequently males



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19331 Tropisms Effecting Copulation in the Bed Bug 119 attempt, often, to copulate with others of their own sex. They even fail to differentiate between mature larvae and adults which are of approximate size and shape. The identical shape of the two sexes in the Japanese beetle, P. japonica, was also the reason for similar behavior in the case described above. The odor may have guided the males to the female; however, upon coming in close contact with each other, they failed to discriminate between the two sexes. The male of the lady beetle, C. 9-punctata, previous to the act of mating, places the forelegs over the middle of the hemi,spherical body of the female and pivots over her, touching with his posterior legs and the tips of his ventrum the margins of the female. One observer relates how a male of the Scarabaeid species, Dynastes tityus L., having been permitted to crawl over his fingers, upon reaching the thumb, oriented himself upon the nail and attempted to insert his genitalia in the space between the nail and skin. One who is familiar with the size and shape of this beetle, and the similarity of the same to a thumb nail, will under- stand the cause of such a reaction.
Reactions of such a nature were designated by early ob- servers as "stereotropic"-meaning reactions to solid ob- jects. This term, the writer believes, is not specific and he suggests the term "morphotaxis7' for the particular kind of reaction discussed previously. This term is more suitable because it is the particular shape of the surface of the fe- male or her specific form which calls forth the definite orientation. Morphotaxis may be considered as a more specific division of stereotaxis. The general term "thig- motaxis," substituting "stereotaxis," the writer believes, is entirely inappropriate for this reason: Most, and perhaps all of the technical names of the various tropisms are based on the Greek name of the stimulus which causes the reaction and not on the name of the sense by which the stimulus is perceived. Thus the terms are "phototaxis" or "chemo- taxis" and not "optiotaxis" or "olfactotaxis.~' Hence, why should "thigmotaxis9'-orientation according to touch- substitute "stereotaxis"-reaction to solid objects? In the case with the above-mentioned Coleoptera, al- though the males manifested definite orientations with re-



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120 Psyche [ December
spect to the characteristic shape of their own species, they lacked the faculty to "recognize" the female even after they came in direct contact with the other individual. The bed bug in this respect is more sensitive. It is true, by the sense of sight it does not distinguish a dead object from a live individual, but, upon coming in contact with it, it imme- diately recognizes the female. As mentioned above, the piece of cork was mounted and dismounted very quickly. The crude carving could not produce the natural shape of the bed bug and the male recognized the imitation very readily. When a male mounts another male, the lower one usually bends the tip of his abdomen upward; he thereby causes a change in his normal shape and form which, in turn, causes the upper male to dismount. Otherwise, the latter orients himself in the characteristic fashion as if he were a female. The shape of the latter stimulates the male to orient himself obliquely as described in the foregoing, rather than laterally. The sense organs of touch along the ventral sides of the last abdominal segments of the male are unequally distributed (Text figure 1). It seems that the shape of the female in this position is so stimulating that even a dead individual calls forth the final copulatory reactions.
Literature Cited.
Barber, H. S. 1929. Personal communications. Cragg, F. W. 1914. A Preliminary note on Fertilization in Cimex. Indian Journ. Med. Res. I1 No. 3, p. 698. ................. 1920. Observations on the Respiratory System of Cimex with Special Reference to the Be- haviour of Spermatozoa. Indian Journ. Med. Res. VIII No. 1, pp. 32-39.
.................... 1925. Observations on the Respiratory System of Cimex. Indian Journ. Med. Res. XII, pp. 451-455.
Hase, A. 1918. Beobachtungen liber den Kopulations- forgang bei der Bettwanze Cimex Zectularius L. Sitz- ungsber. Ges. naturf. Freunde, Berlin. VIII, pp. 311-321.
Mayer, A. G. 1900. The Mating Instinct in Moths. Psyche, IX, pp. 15-20.




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