The Colouration of Insects. 77 



nearly all moths ; and in the other orders of winged insects the 

 decoration is even more pronounced, as any one may see who looks 

 at our dragon-flies, wasps, bees, or even beetles. 



In some insects the decoration of the body is very marked, as in our 

 small dragon-flies, the Agrions. In one species, for example, A. Puella, 

 the male is pale blue banded with black, and the female bronze 

 black, with a blue band on the segment, bearing the sexual organ ; 

 the ovipositors are also separately decorated, The male generative 

 organs are peculiar, in that the fertilizing fluid is conveyed from one 

 segment to a reservoir at the other end of the abdomen. Both the 

 segments bearing these organs are marked by special decoration. 

 The peculiar arrangement of the sexual organs in dragon-flies is very 

 variable, and certain segments are modified or suppressed in some 

 forms, as was shown by J. W. Fuller.* In every case the decoration 

 follows the modification. In the thorax of dragon-flies, too, the 

 principal muscular bands are marked out in black lines. This 

 distinct representation of the internal structure is beautifully shown 

 in ^Eschna and Gomphina, and in the thorax of Cicada, as shown 

 by Dr. Haagen in the paper quoted in the last chapter. 



We may, then, safely pronounce that the decoration of insects is 

 eminently structural. 



Simple Variation. Cases of simple variation have been already 

 cited in our description of spots and stripes, and it only remains to 

 show that in this, as in all other cases, the variation is due to a 

 modification of original structural decoration. 



To take familiar examples. Newman, in his British Butterflies, 

 figures the varieties of the very common Small Tortoiseshell ( Vanessa 

 urticcB). In the normal form there is a conspicuous white spot on 

 the disc of the fore-wings, which is absent in the first variety, owing 

 to the spreading of the red-brown ground colour. This variety is 

 permanent on the Mediterranean shores. In variety two, the second 

 black band, running from the costa across the cell, is continued 

 across the wing. The third variety, Mr. Newman remarks, is 

 " altogether abnormal, the form and colouring being entirely altered." 

 Still, when we examine the insect closely, we find it is only a modi- 

 fication of the original form. The first striking difference is in the 

 margin ot the wings, which in the normal form is scalloped with 

 scallop-markings, whereas, in the variety the margins are much 

 simpler, and the border pattern closely corresponds with it, having 



* J. W. Fuller on the Breathing Apparatus of Aquatic Larvae. Proc. Bristol 



Nat. Soc. 



