BY R. J. TILLYARD. 305 



ground colour has been modified ; whereas in the case of /. 

 heterosticta the 9 Form B imitates the (J almost completely.* 



The only possible explanation seems to be as follows : — The 

 genus Ischnura contains, of all the Agi'ioriidce, some of the most 

 fragile and defenceless insects. Their powers of flight are 

 usually confined to skimming from reed to reed over the' surface 

 of the water, rarely more than an inch or two above it, or hover- 

 ing in the sun for a short period. Hence they fall an easy prey 

 to the more rapacious species that roam over the ponds and 

 rivers. It seems probable that the second form of 9 may in 

 some manner help to prevent the extermination of the species by 

 deceiving its enemies. For the female, having a more robust 

 abdomen than the male, would appear a far more preferable and 

 palatable morsel, so that the species might easily be threatened 

 with extinction by the destruction of the ova-laden females. 

 The second form of the female might be overlooked owing to its 

 difference of colour, and thus escape destruction. 



Now /. heterosticta is remarkable in being a larger and more 

 active insect than the other species of the genus. The develop- 

 ment of the two forms may therefore, in the more robust species, 

 have reached beyond a mere change of ground colour and involved 

 a change in the shape of the abdomen also. It is at any rate 

 remarkable that the change in this case is one of almost com- 

 plete mimicry of the male, suggesting that the male is regarded 

 as an unpalatable and worthless capture by its enemies in com- 

 parison with the fat and well-favoured female. It is also worthy 

 of note that so far no second form of the female has yet been 

 found in any other Australian species of Ischnura. The small 

 size of most of the other species is perhaps a sutHcient protection 

 against their enemies. 



* See also " Entomologist," Vol.xxxvii. p. 252, where is described a form 

 of the $ of Ischnura elegans, in which the ordinary blue parts are replaced 

 by dull olive. This, however, appears to be only a local variety. It is 

 interesting to note that the writer, F. W. Campion, considers it a reversion 

 to an ancestral type. — R. J. T. 

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