11 



the forms of objects, or at best, very poorly. The antennae or feelers 

 have imp rtant functions, probably connected with motion, touch, 

 hearing, taste, or smell. 



The external covering of all insects, and indeed of all the Articxdata 

 or segmented animals, is a substance, usually more or less hardened, 

 known as chitin. It is an excretion from the soft underlying 

 cellular layer, the epidermis. In the burrowing earthworms a pro- 

 tective covering is not much required, and the cuticle is very thin and 

 delicate. In crabs and lobsters it becomes impregnated with lime salts, 

 and consequently very hard and a most effectual protection. This 

 would, however, be too heavy for the air-disporting insects. It is to be 

 noticed that the chitinous case is not stiffened at the joints, so that 

 movement of one part of the body upon another is allowed. The outer 

 skin becoming hard and inelastic soon after exposure, the necessity for 

 the creatures, while increasing in size, to be occasionally moulting iheir 

 skin as a whole is obvious. 



The internal structure of insects can best be studied on such a form 

 as our common large cockroach. Although a very objectiouable and 

 much despised creature the cockroach is worthy of our great respect 

 on account of its long ancestry. We occasionally give honour to indi- 

 viduals who can boast only of this as their recommendation, and it is 

 certainly one redeeming feature in the present case. Forms similar to 

 the cockroach are met with amongst the early fossiliferous rocks, and 

 extend, unquestionably without any signs of extinction, down to the 

 present day. 



On pinning down a cockroach and cutting away the upper portion of 

 the body- wall the interior is seen almost filled with a white loose 

 substance, which is mainly fatty matter held together by the fine air- 

 tubes or tracheae. When this is cleared away a partially coiled tube is 

 seen extending the whole length of the body. . It is the food-canal of 

 the animal and is divisible into different parts, each with a separate 

 function in the complex process of digestion. Outgrowths from it occur 

 at different places; two in front are known as salivary glands and pro- 

 duce a digestive fluid, and two other groups occur behind. The first, 

 consisting of short pouch-like tubes, secretes another digestive fluid, and 

 the second group of long slender tubes has an excretory function. The 

 dorsal heart and the blood vessels are not readily seen. Run 1 ing all the 

 length of the ventral surface of the body-cavity, which is not however 

 a true ccelome, is the double nerve-cord, with enlargments corresponding 

 somewhat with each segment and from which are given off fine 

 branches to all parts of the body. The complex reproductive organs 

 are seen behind. 



"With some interesting exceptions, all insects develop from eggs. 

 The life of most may be divided into three stages : the larva, caterpillar, 

 or grub; the pnpa or chrysalis; and the imago or adult ; the change 

 from one to the other being spoken of as a metamorphosis. The differences 

 between the various stages are however very diverse in the many groups ; 

 the obvious alterations being much more marked in some than in others. 

 Illustrative examples were given from slides showing the life-history 

 of the silk worm, Bombyx; the Water-beetle, Dytiscus ; and the Hawk- 

 moths, Sphiiigidce. 



