著者

小松 謙之

出版者

麻布大学

巻号頁・発行日

2015

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The so-called “Ogasawara cockroaches”, which hitherto has also been synonymously called “Surinam cockroaches”, are widely distributed throughout the world in the tropical and sub-tropical areas. In Japan, their distribution is limited to Kagoshima in Kyushu and the island chain of Nansei and Ogasawara island chain. In recent years, this cockroach has invaded into urban buildings and abode, resulting in the report of their capture or eradication attempts. The problem of urban buildings invasion by this cockroach has also been seen in other countries. From an epidemiological perspective, Kim and Erko (2008) reportedly detected the potentially zoonotic helminth ascarid and taeniid eggs on the body surface of this species of cockroach in Ethiopia. Furthermore, they also reported the presence of trichurid eggs and Entamoeba coli cyst in the digestive tract of the cockroach. These findings has put the spotlight on the role of the cockroach as a potential mechanical transmitter zoonotic infectious diseases.Regarding the constituent species of the so-called “Ogasawara cockroaches” or “Surinam cockroaches”, it seem that there were two apparently morphologically similar species, namely Pycnoscelus indicus and Pycnoscelus surinamensis, with the former showing bisexual reproduction and the latter, parthenogenesis. For the identification of these two species, Roth (1967) proposed that for the adults, irrespective of the presence or absence of sperms in the spermatheca, those which produce only female offspring should be identified as Pycnoscelus surinamensis, while those that produce both male and female offspring should be relegated as Pycnoscelus indicus. Moreover, he also reported that there were morphological differences between the two aforementioned species, based on the distance between the compound eye and ocelli. In P. indicus, the ocelli and the compound eyes were separated, whereas in P. surinamensis, the two eyes were in contact.The male of P. indicus are relatively more susceptible to dryness than the female, and easily died off under non-optimal environmental condition, leading to reduced chances of mating. However, since P. surinamensis is parthenogenetic and does not need male to reproduce, they can even proliferate under a harsh environmental condition if a single female is able to invade and survive under that condition.Asahina (1991) reported that both male and female individuals could always be found among the “Ogasawara cockroaches”, without noting that the female were parthenogenetic. This led him to suspect that the Japanese “Ogasawara cockroaches” might include P. indicus. However, he proposed that until the detailed breeding experiments had been carried out, the “Ogasawara cockroaches” should all be tentatively be identified as P. surinamensis. Thus, despite that the Ogasawara cockroaches has been recognized as a pest, their species identification and distribution has not been clearly elucidated.To clarify the species involved, first of all, we need to establish a criteria be able to identify the sex of the cockroach at the larval stage before they mature into set out to establish a method to differentiate the sex of male and female cockroach from the larval stage to the adult stage. Sexual differentiation of the cockroach nymph for all the instar stages of Blatta orientalis, Supella longipalpa and Periplaneta fuliginosa has been established, based on the morphological observation of the abdominal segments of the in star.In our study, cockroaches were collected in Ishigaki island, Taketomi-cho, district, Okinawa prefecture, Japan. They were then reared and passaged for several generations in the laboratory. Only those groups that produced offspring, which matured into male and female adults were used in the following experiment. The instar that hatched from the eggs were immediately isolated and separated according to their morphoplogical characteristic of ventral segments. Changes on the ventral segment were also noted for each instar stages and ultimately, the sex of the mature adult for each group was determined. Using the aforementioned method, the female of the 1st to 6th stage instar nymph were found to possess a V-shaped notch at the middle of the posterior edge of the 9th sternite. This notch was not seen in the male nymph. In the female 7th stage (final stage) instar nymph, the styli were not apparent and, the 8th and 9th sternites became degenerated and were covered over by the profoundly developed 7th sternite. In contrast, all stages of the male nymph until the 7th stage nymph showed the presence of the 8th and 9th sternitesas well as styli. Based on these observations, our study has demonstrated that it is possible to differentiate the sex of the Indian cockroach, P. indicus, at different developmental stages.Next, to identify the various specimens as to which stage the nymph instar belong to, we counted the number of cercal segments from the dorsal and ventral view. It was observed that the number of cercal segments from the dorsal view in 2nd and 3rd stage nymph were the same, that is 4 and thus could not be used to identify the nymph stage. However, when viewed ventrally, it was observed that the number of cercal segments on the1st stage nymph was 3; 2nd stage, 4; and in the subsequent stages, an increase of one extra segment for each stage. The number of cercal segments of all the stages of the female, right up to the 7th stage nymph, when viewed ventrally were the same as that of the male nymph. Therefore, the developmental stage of the nymph could be identified by examining the number of segments from the ventral view.Based on the above results, we set out to confirm species and distribution of Surinam cockroaches inhabiting Japan. We collected cockroaches from Ogasawara island chain (Iwoto, Hahajima, Chichijima, Nishijima, Nakodojima), Amami island chain (Tokunoshioma, Amamiooshima), Okinawa island chain (Okinawato, Miyakojima, Ishigakijima) and Hawaii, kept the female in solitary rearing for use in later mating experiments. Cross breeding experiments that were carried out showed that there were those cockroaches from Iwoto, Tokunoshima, and Okinawato that produced only female offspring, and also that produced both male and female offspring. The former group was designated A group and the latter, B group. Both groups comprises of 14 isolates from 11 areas. The cockroaches from both groups were then used for subsequent cross breeding experiments. For the cross breeding experiments, those specimens that produced only female offspring were mated with those male from Hawaii that had produced both male and female offspring and had been identified a P. indicus as previously reported by Roth (1967). From the results of our study, the area that contain the two different characteristic types of the cockroaches were Iwoto, Tokunoshima and Okinawato, whereas an individuals of Iwoto- A group, which produced a total of 478 female and no male offspring, despite having sperms in their spematheca, can be identified as P. surinamensis. On the contrary, while those of Iwoto-B group with all having sperms in their spermatheca, produced a total of 168 male and 157 female offspring in an average ratio of 10.8 to 10.5 (p>0.05) with no significant difference in the sexual ratio, can identified as P. indicus. Cockroaches of Tokunoshima-A group that produced a total of 221 female and no male offspring, despite having sperms in their spematheca, can be identified as P. surinamensis, while those of Tokunoshima-B group with all having sperms in their spermatheca produced a total of 242 male and 207 female offspring in an average ratio of 12.1 to10.4 (p>0.05) with no significant difference in the sexual ratio, can thus be identified as P. indicus.On the same note, female cockroaches of Okinawato-A group that produced a total of 724 female and no male offspring, despite having sperms in their spematheca, can be identified as P. surinamensis, while those of Okinawato-B group with all having sperms in their spermatheca produced a total of 322male and 312 female offspring in an average sexual ratio of 16.1 to 15.6 (p>0.05) with no significant difference in the sexual ratio, can be identified as P. indicus.Thus, both species of P. surinamensis and P. indicus were found to be distributedon the three islands of Iwoto, Tokunishima and Okinawa, with their habitat overlapping with each other.The group of five F1 female cockroaches from Hahajima, Chichijima, Nishijima and Nakodojima, produced a total of only 248, 59, 663 and 143 female offspring, respectively and no male offspring, despite having sperms in their spematheca. These cockroaches were identified as P. surinamensis. Thus, there is a possibility that only P. surinamensis and not P. indicus are distributed on these four islands.The group of five F1 female cockroaches from Amamiooshima, Miyakojima, Ishigakijima and Hawaii, produced a total of 260 male (M) and 260 female (F) offspring with an average ratio of M:F at 14.4:14.4 (p>0.05) per litter, 230M, 267F, av. 16.4:19.1 (p>0.05) per litter, 281M, 266F, av. 16.5:15.6 (p>0.05) per litter and 199M, 189F, av. 11.7:11.1 (p>0.05) per litter, respectively. Thisprobably indicates that only P. indicus and not P. surinamensis were inhabiting the four islands.From the above results, we can conclude that there are areas in Japan where the distribution of P. surinamensis and P.indicus overlap with each other, and there are also areas in which either only one or the other could be found.Futhermore, all the various specimen from our study were examined for morphologic differences between the 2 species. Roth (1967) stated that P. surinamensis could be morphologically distinguished from P. indicus based on the distance between the ocelli and compound eye, in which the former species show contact between the ocelli and the compound eye, while in the latter species, they are separated. However, in our experiments, we could not find any female adult cockroach of P. surinamensis, whose ocelli were in contact with the compound eye, that is, for the groups that do not produce any male offspring, the distance between ocelli and compound eye in the adult female from the various localities are as follows: Hahajima, 0.16 mm > Chichijima, 0.14 mm > Nakodojima, 0.13 mm > Nishijima, Tokunoshima-A & Okinawato-A, 0.12 mm > Iwoto-A, 0.10 mm, respectively, with an average distance of 0.13 mm. For the groups that produce both male and female offspring, identified as P. indicus, the distance between ocelli and compound eye in the adult female from the various localities are as follows: Hawaii, 0.21mm > Iwoto-B, 0.18 mm > Miyakojima, 0.16 mm > Amamiooshima & Okinawato-B, 0.13 mm > Tokunoshima-B & Ishigakijima, 0.12 mm, respectively, with an average distance of 0.15 mm.There was no significant difference in the distance between the ocelli and compound eye between the two species. Thus, this morphological criterion is not applicable for species identification.The length of tegmina has been used as a criterion for species identification in many insects. Thus, we proceed to measure the length of the tegmina of the adult female cockroaches in our study. For the groups that do not produce any male offspring, identified as P. surinamensis, the average tegmina length of the adult female from the various localities are as follows: Okinawato-A, 15.82 mm > Hahajima, 15.26 mm > Nishijima, 15.07 mm > Nakodojima, 14.16 mm > Chichijima, 13.81 mm > Tokunoshima-A, 13.57 mm > Iwoto-A, 12.87 mm, respectively. For the groups that produce both male and female offspring, identified as P. indicus, the average tegmina length of the adult female from the various localities are as follows: Okinawato-B, 14.72 mm > Hawaii, 14.64 mm > Iwoto-B, 14.35 mm > Ishigakijima, 13.81 mm > Tokunoshima-B, 13.54 mm> Amamiooshima, 13.53 mm > Miyakojima, 12.96 mm, respectively. It was observed that there was not much difference in the tegmina length among the specimens from different localities and also between the two species, thereby excluding the used of this criterion for species identification.Moreover, to differentiate the species without using the cross breeding experiment, we tried the method of solitary rearing of the unmated female adult cockroaches obtained from our previous experiments. Our results showed that all individuals identified as P. surinamensis produced offspring, and based on our previous sexual differentiation method of the nymph, all the offspring nymph were found to be female. On the contrary, individuals identified as P. indicus did not produce any offspring. From this observation, we can identify the species of the adult female cockroach collected from the wild by solitary rearing and examining the sex of the offspring produced. Thus, those female adult that produce only female offspring can be identified as P. surinamensis and that that produce both male and female offspring or those that did not produce any offspring can be identified as P. indicus.Based on the results of the above studies, the so-called “Ogasawara cockroaches” which has until now been thought to consist of only P. surinamensis, actually also comprises of P. indicus, which are also distributed in the same area. From our study, we have also added one more species of cockroach that inhabit Japan, that is to 58 species. Until recently, P. indicus and P. surinamensis were thought to be distributed in different areas but our study shows that there are areas in which both species co-habitat together and there are also areas in which either only one of the two species can be found. This finding has deep implication for future studies. In addition, we also found that the criterion of using the distance between the ocelli and the compound eye for species identification, as proposed by Roth (1967), is applicable nor reliable. We propose an alternative method in the form of solitary rearing of wild female adult and determining the sex of the offspring for species identification. Those that produce only female offspring be identified as P. surinamensis, while those that produce both male and female offspring, as well as that that failed to produce, should be regarded as P. indicus. Since the report by Asahina (1991) that species identification of “Ogasawara cockroaches”, needs further clarification, the results of our present study has provided the answer to his question through our morphological observation, cross breeding and solitary rearing experiments of those cockroaches.