266 THE MECHANICAL FUNCTIONS. 



formed for a terrestrial, instead of an aquatic life. 

 Most of the lower tribes of this order are parasitic, 

 that is, derive their nourishment 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 remark- 

 able circumstances attending their mechanical 

 conformation. 



The Pulex, or flea, is the only apterous insect 

 which undergoes complete metamorphoses in the 

 course of its developement.* In the first stage of 

 its existence, it has the form of a long worm, with- 

 out 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 mechanism pervad- 

 ing even the lowest orders of the animal creation. 



natural affinities. Latreille establishes four orders of wingless insects; 

 namely, Myriopoda, Thysanoura, Parasitica, and Sugeiitia. Cuvier, 

 Regne Animal, 2nd edition, 1829. 



* Accordingly Lamarck, in the last edition of his " Histoire 

 Naturelle des Animaux sans vertebres," considers the order Aptera 

 as formed by the genus Ptdex alone. Tom. iv. p. 7. 



