382 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



show that the absolute lengths are decidedly greater in the Agri- 

 oninae than in the Libelluhnae. The number of species dealt 

 with is of course too few to afford a basis for establishing the 

 average or the mean for each subfamily, but there can be no 

 doubt of a very considerable percentage of difference between 

 these groups, especially the extremes. By reference to the ratios 

 between the length of the synthorax and that of the abdomen it 

 is just as clear that the latter has decreased in i^elative length in 

 the higher groups. For example: the abdomen of Calopteryx 

 maculato. cf measures 35 mm. while that of Tramea Carolina cf 

 is but 28 mm. The synthorax of these species is respectively 

 5.0 mm. and 9.5 mm., giving a ratio of thorax to abdomen of 1:6 

 in the former and 1:3 in^the latter. If a third example is selected 

 from the lower Anisoptera, Hagenius brevistijhis or Gomphus 

 cornutus, an increase over Calopteryx in absolute length may be 

 seen, but the length relative to the synthorax gives the ratio 

 1:4, which stands intermediate between 1:6 and 1:3. Very 

 roughly speaking thoracic-abdominal ratios for each of these 

 selected subfamil}^ groups approximate the following: Agri- 

 oninae 1:6; Lestinae 1:5.5; Coenagrioninae 1:5; Gomphinae 

 and Aeshninae 1: 4.5; and Libelluhnae 1:3. A valid generaliza- 

 tion could only be reached after a much more extended survey. 

 The Pseudostigmatinae can here be represented by but two 

 species, Mecistogaster modestus and Megaloprepus coerulatus with 

 ratios ( 9 ) of 1 : 10 and 1 : 8.9. It may be of value to note two species 

 of about the same body length, thorax plus abdomen: Hetatrina 

 americana and Pantala hymenaea measure about 40 mm., but the 

 abdomen of the former is 34 mm. and that of the latter 29 mm.; 

 the thorax is 6 ram. and 10.5 mm. respectively. Thus a reduc- 

 tion of about one seventh of the abdominal length is accompanied 

 by an increase of almost two times in the thorax. The thorax 

 of Pantala is also much larger in other dimensions. 



The sexes vary in the size of the abdomen. Another examina- 

 tion of the table will show that the male is generally 1 or 2 mm. 

 longer than the female, as well as being more slender. Infre- 

 quently the difference is as great as 3 mm. In the Lestinae the 

 females of the various species on our list are often so nearly alike 

 both in coloration and size as to make them inseparable in the 

 absence of the males. But when we include the males also, the 



