APTEROUS INSECTS. 297 



order are parasitic, that is, derive their nourish- 

 ment from the juices of other animals, the skin 

 of which they infest and penetrate, and into 

 which they insert tubes for suction. The various 

 tribes of Acari, or mites, of Pediculi, or lice, of 

 Ricini, or ticks, of Pulices, or fleas; together with 

 the Podura, or spring-tail ; the Lepisma, and 

 the family of Myriapoda, or millepedes, are 

 comprehended in this order. I shall be obliged 

 to pass over these tribes very cursorily, noticing 

 only a few of the more remarkable circumstances 

 attending their mechanical conformation. 



The Pulex is the only apterous insect that 

 undergoes complete metamorphoses in the course 

 of its developement. In the first stage of its 

 existence, it has the form of a long worm, without 

 feet, frequently rolling itself into a spiral coil. 

 It consists of thirteen segments, having tufts of 

 hair growing upon each. In its mature state it 

 has six articulated legs, the hindmost of which 

 are of great size, for the purpose of enabling the 

 insect to take those prodigious leaps which 

 astonish us in beings of so diminutive a size, and 

 afford a striking proof of the exquisite mecha- 

 nism pervading even the lowest orders of the 

 animal creation. 



The Podura leaps into the air by a mecha- 

 nical contrivance of another kind ; employing 

 for this purpose the tail, which is very long, and 

 forked at the end. In its ordinary state this 



