586 PHIL(3S0PHY OF ZOOLOGY. 



but which do not agree with any of the preceding classes. 

 They possess one common character in wanting wings, in 

 all the stages of their existence. They may be divided in- 

 to three orders, which by some are elevated to a primary 

 rank. 



I. Apteiia Suctoria. — The head, thorax, and abdo- 

 men intimately united. The mouth consists of two simple 

 processes, the lowest of which is longest, and receives the 

 superior one in a cavity in its upper side. On each side is 

 a process of four joints covering the others. These unite 

 to form a proboscis, which rests upon the sternum. Are 

 the simple processes to be considered as the labrum and la- 

 bium, and the articulated lateral ones as palpi, the mandi- 

 bulae and maxillae being absent .'' The antenna consist of 

 four joints. This order contains at present only one genus, 

 Pulex or Flea. There are several species. The P. irri- 

 tans is every where common, and the P. fasciatus of Bosc 

 may be met with on moles and mice. 



II. Aptera Thysanura.— The head in this and the 

 following order is obviously separated from the thorax by 

 a contraction or neck. The last segment of the body is 

 furnished with long filaments. In the Lepismada the se- 

 tag of the tail are continually extended in the direction of 

 the body as Lepisma and Forbicina. In the Poduradae, 

 the setae, when at rest, are folded under the body, as Po- 

 dura and Smynthurus. 



III. Aptera Parasita. — The tail in this order is 

 simple, or destitute of the filaments which distinguish the 

 preceding. The Nirmidas, including numerous species of 

 lice which infest birds, and Included in the genera Nirmus * 



• See figures of three species found on the Anser erythropus, 9th Janu- 

 aiy 1819, Plate iv, f. 6. a. b, c. 



