ACARIXA PARASITES. 327 



shaped mandibles, there is a complete system of tracheae with spiracles, 

 as in true insects. Besides these two there is an intermediate plan of 

 respiration not known before, combining both the other modes, in 

 which inspiration takes place through the skin, and expiration through 

 a system of tracheae, which have their outlet above the insertion of the 

 mandibles. " Trombidiun is an example in which a latticed aperture at 

 the root of the mandibles forms the anterior outlet of two large air- 

 pipes running from the hinder end to the front, each subdivided into a 

 tuft of numerous unbranched simple tracheae. Besides these, there is 

 under the skin a round meshed network of a transparent and seemingly 

 homogeneous substance, resembling the respiratory network under the 

 skin of certain Trematoda. 



" The importance of a thorough examination of these microscopic 

 pests is at once evident, in the fact that the type of the family to which 

 the whole of them belong is the noisome parasite of the human subject; 

 that another, as yet undetermined form of the same tribe, is thought by 

 some to be connected with one of the most fatal ailments of our frame 

 dysentery ; that two distinct Sarcoptes, yet undescribed, affect the 

 horse and sheep ; and that even the common sparrow, our little pet 

 canary-bird, and even the useful bee, do not escape the ravages of one 

 of the family. 



When, therefore, we reflect on the ailments which these produce, 

 and on the diminutive size of the creature which in its effects is so 

 destructive to other tribes ; and bear in mind that this mere speck, 

 this particle of dust, is organised for all its purposes as completely as 

 the most perfect of any of the whole sub-kingdom to which it belongs, 

 even to the flexor, the extensor, and the rotator muscles of its truly 

 atomic limbs ; while the entire body of the creature, when first pro- 

 duced, measures scarcely more than the sixteen -thousandth of an 

 inch in length ; and then call to mind that the mere foot of the Di- 

 nornis, or of the Palapteryx, the ancient colossal bird of the antipodes, 

 measures, as shown by Professor Owen, more than seven hundred and 

 fifty times the whole size of this little body, who can but feel astonished 

 at this range of creation 1 Who can but feel that the study of natural 

 history, not as the amusement of an hour, but as a sober contempla- 

 tion, must tend to exalt as well as to extend the human intellect, and 

 that the most microscopic atom of organised life, considered as part of 

 the world, is as deserving of our fullest attention as the most gigantic V 3 * 



The Louse (Plate XI. No. 1). Whenever wretchedness, disease, and 



* George Newport, Esq., F.R.S., "On a new genus of the family Chalcididce, found 

 n the nest of the bee." Linnean Society's Transactions, 1853. 



