APHID^E. 521 



MM. Balbiani and Signoret have discovered that the com- 

 mon European Aphis aceris produces, besides young of the 

 normal form, a singular dimorphous form (Fig. 517), first de- 

 scribed in 1852 by Mr. J. Thornton, under the name of Pliyl- 

 loplwrus testudinatus, and afterwards called Periphyllus testudo 

 by M. Van der Hoe veil. The chief characteristic of this re- 

 markable form, which is flattened, scale-like, is the series of 

 leaf-like scales surrounding the body and bordering the appen- 

 dages, while the upper side of the abdomen is covered with 

 hexagonal figures. The generative apparatus is also vei'v ru- 

 dimentary. It does not produce young, -and the insects them- 

 selves do not increase in size after birth, being scarcely one 

 millimetre in length. "They undergo no change of skin, 

 never acquire wings like the reproductive individuals, and their 

 antennae always retain the 

 five joints which they pre- 

 sent in all young Aphides 

 before the first moult." 

 (Science Gossip, 18G7, p. 

 204.) 



Aphides are found upon 

 every part of plants. 

 Some species which are rig. 518. 



wingless, are found on the roots of plants, others on the stems 

 of twigs, others roll up leaves, or form gall-like swellings on 

 leaves ; the grain Aphis sucks the sap of the kernel. Ants are 

 fond of the sweet excretions from the abdominal tubes, and 

 often keep them captive in their nests like herds of cattle. 

 Syrphus flies, Cocciuellae, etc., keep them within proper limits 

 in nature. Various species of Aphidius kill larger numbers 

 than we imagine. "When an Aphis has received an egg from 

 one of these parasites it quits its companions and fastens itself 

 by its ungues to the under side of a leaf, when it swells into a 

 globular form, its skin stretched out and dried up, and in a 

 short time the perfect parasite escapes by a circular hole, the 

 mouth of which sometimes remains like a trap door." In 

 the Museum of the Peabody Academy is an apple twig almost 

 covered with dead Aphides, each perforated by a hole from 

 which an Aphidius had escaped. 



