354 LECTURE XVI. 



insects devoid of wings have been artificially grouped together. Cu- 

 vier and Latreille divide the Apterous Insects into three tribes, the 

 Suctoria (fleas), the Parasita (lice, including the Pediculus capitis 

 and Pthirus ingiiinalis of the human species), and the Thysanoura, 

 including the Lepisma and Podura or skip-tails. 



The grand and characteristic endowment of an insect is its wings ; 

 every part of the organisation is modified in subserviency to the full 

 fruition of these instruments of motion. In no other part of the 

 Animal Kingdom is the mechanism for flight so perfect, so apt to 

 that end, as in the class of insects. The swallow cannot match the 

 dragon-fly in its aerial course ; this insect has been seen to outstrip 

 and elude its swift pursuer of the feathered class : nay, it can do 

 more in the air than any bird, — it can fly backwards and sidelong, to 

 right or left, as well as forwards, and alter its course on the instant 

 without turning. 



Now what are these "limber fans," that give the little articulate 

 animals such command over aerial space ? I do not mean their struc- 

 ture or composition ; the anatomical question has been already 

 answered. I do not ask for their analogy ; that is riglitly expressed 

 by their common name ; they have the same relation to the insect as 

 instruments of motion, which the feathered wing bears to the bird. 

 But what is their essential nature, or with what are the wings of the 

 insect homologous? Are they modified anterior limbs, like the wings 

 of bats and birds and flying-fishes ? Not so, for they co-exist with 

 and are superadded to the jointed anterior pair of legs. Are they 

 such expansions as form the parachute of the little dragon {Draco 

 volans) ? These do, indeed, co-exist with arms and legs, but they 

 consist of a fold of integument stretched out upon elongated and 

 straightened ribs, which are appendages of a vertebral column. 

 But an insect has no vertebral column, no true internal skeleton. 

 The strong and numerous nervures which sustain the thin alar mem- 

 branes of the Libellula are articulated processes of the external chi- 

 tinous tegument. 



A cii'culation can be traced through these membranes, at least in 

 their early and softer state ; air-vessels are abundantly spread over 

 the supporting frame-work : the wings of the Lepidoptera appear 

 after the third moult, as tegumentary flattened vesicles, soft, and 

 permeated by trachea?, and when fully expanded in the imago, they 

 must still take their share in the business of respiration. Nay, it has 

 been found that the rudimental wings of the pupas of certain water 

 insects are the gills of such ; they perform the same function as the 

 very similar membranous and vascular tegumentary expansions in 

 certain Anellides (see ,/f^. 101,); which expansions are developed, 



