Studies on the age of mosquitoes : II. Comparison of three determination methods, the relic observation, the tracheole method and the granule technique

Abstract

The age determination of mosquitoes was carried out with the following populations, first was Culex pipiens fatigans of indoor collections with insect aspirators at night in May 1963 from each house at a certain village in Okino-Erabu Island, another was Aedes togoi also of night collections in August 1963 at houses in Hachijo-Koshima Island. The mosquitoes were kept alive in small cages until the dissections were done next morning. Out of 333 mosquitoes of Culex pipiens fatigans which were examined about the relic numbers of ovarioles, 174 individual or 52.3% were nulliparous, 127 (38.1%) uniparous, 29 (8.7%) biparous and 3 (0.9%) were found triparous. With Aedes togoi, 48 (44.0%) out of 109 individuals were nulliparous, and 50 (45.9%) uniparous, 10 (9.2%) biparous and 1 (0.9%) triparous were observed. The determinations by tracheoles after Detinova (1949) and by observing of granules according to Colles (196) were conducted in parallel with the examination of relics in each individual of mosquitoes. Either tracheole method or granule method yielded almost the same parous rate as that by relic method. However, in a considerable part of specimens the three methods did not always give a good concordance. It was shown that tracheole method would lead to false positive results particularly in the specimens of which the follicles developed. Because the enlargement of ovaries even of nulliparaous mosquitoes result the unfolding of tracheole ends. In the use of granule method the pigment of ovarian sheath was often a trouble and did mislead to false positive results. In both of two methods other than the relic method the yolk in too much developed follicles, that is, later than the 3rd stage by Christophers' criteria made the observations either of tracheoles or granules extremely difficult. Therefore a large part of materials were left undetermined of age by these method, whereas the relic method was available. Besides the relic method only can determine the number of oviposition and give more informations of physiological age, so other two techniques should be adopted only in auxiliary use, although they have an advantage in the examination of a large number of materials in a day.