HOMOPTERA. 307 



naturally in the air," while another philosopher gives it 

 credit for breeding the very insects which produce it. 



The presence of Aphides raay be detected in cases 

 where the insects themselves are concealed, by various 

 effects produced upon the plants infested by them. Some 

 species cluster on the under-side of leaves, either form- 

 ing numerous little concave nests in the under-side, while 

 the upper rises into corresponding convex excrescences, 

 or curving the whole leaf into one mighty dome in which 

 many hundreds live. Others, sucking the juices from 

 the stems of plants, cause contractions and distortions of 

 various kinds. Others, again, form large gall-like 

 excrescences upon various parts of plants. Of these the 

 pear-shaped sacs on the leaf-stalks of the Lombardy 

 poplar are a common example. 



A very curious little insect, supposed to be the young 

 of some species of Aphis,* may be found on the under- 

 side of maple and sycamore leaves, generally (except 

 when numerous and scattered all over the leaf) sheltering 

 itself in the angle formed by two veins. To the naked 

 eye it is a minute green, or brownish-green scale ; under 

 the microscope it is one of the most singular-looking 

 creatures possible. This atom rejoicing in the names of 

 Chelymorpha phyllophorus, or " the leaf-bearing tortoise- 

 shaped," and Phyllophorus testudinatus, or " the tortoise- 

 like leaf-bearer" (and if a third variety could be formed by 

 twisting the name any other way, the insect is quite worthy 

 of the honour) is a flat, tortoise-shaped green insect, 

 bearing on its head a crown or tiara composed apparently 

 of four beautiful leaves, as clear as glass, and delicately 



* It much more nearly resembles the young of some other insects in 

 the Order. 



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