Sex ratios in Odonata larvae, with particular reference to the Zygoptera

Larval sex-ratios were investigated in populations of Pyrrhosoma nymphula (Sulz.) and Coenagrion puella (L.) from a point when larvae were approximately half-grown until just prior to, or including adult emergence. In both species there was a very small (averaging 52.0 and 54.4% respectively) but statistically significant excess of male larvae present throughout this period of development. These results are compared with other work on Odonata sexratios. Including the present study, an excess of male larvae is reported in eight of the available records for Zygoptera, and this imbalance is statistically significant in five of the records, involving two species. Another five records for the Zygoptera show an excess of female larvae, with the imbalance being statistically significant in two cases. There is an indication that sex-ratio imbalances involving an excess of females are larger than those involving an excess of males. These results are in direct contrast to the situation in the Anisoptera, in which all twenty-one of the available records show an excess of female larvae. The reasons for the difference between Anisoptera and Zygoptera in this respect are uncertain; nor are the causes of the sex-ratio imbalances in Odonata understood.