DIGESTIVE SYSTEM. 63 



take part in their structure. Tracheae densely 

 ramify in these tubercles, and a large nerve is 

 supplied to them. M. Leydig compares these fleshy 

 plates to the tracheal lamellae forming the rectal 

 branchiae, or gills, of the aquatic larvae of the 

 libellulidae, or May flies. 



Embryogeny (the science of the development of 

 embryos) shows that, whilst the bodies of insects 

 are being fashioned within the egg, the digestive 

 tube is formed in three parts, as follow : The 

 mouth and anal extremities by invaginations (see 

 page 34) of the external skin, the central portion 

 from a modification of the yolk sac, which is met 

 at its ends by the continued deepening of the 

 posterior and anterior invagination. The parts have 

 now no communication, their ends being blind, 

 but are placed like two half sausage skins, with 

 a whole skin in the centre. The separating walls 

 at length undergo absorption, and the tube be- 

 comes single and united, passing through the body 

 from end to end ; the annexed organs are then 

 marvellously added to this simple tube, which forms 

 gradually, at definite spots upon its wall, prolonga- 

 tions, at first like the fingers of a glove, but by 

 degrees assuming the involved structure they possess 

 in the adult insect. In this manner within the 

 bee egg the spinning glands (Fig. 15), which 

 subsequently become System 3 of the salivary 

 elands (Fig. 16), are formed at the anterior part, 

 and at the posterior, similarly, the Malpighian, or 

 urinary tubes. An arrested development, however, 

 in bees, hornets, and wasps, causes the middle 



