330 



GENERAL BIOLOGY 



However, those insects living in water have tracheal or blood gills, 

 or both, or at least some specialized adaptation by which oxygen may be 

 used. 



A peculiar feature of all animals possessing an exoskeleton is, that 

 as soon as the inside of such skeleton grows but slightly, it becomes too 

 large for its skeletal jacket, so that it must split and a new one form. This 



A.-D. Successive stages in the concentration of the central nervous system of 

 Diptera. A, Chironomus; B, Empis; C, Tabanus; D, Sarcophaga. (After Brandt). 



E. Internal anatomy of Calopte'nus fc'mur-rubrum: at, Antenna and nerve 

 leading to it from the "brain" or supra-esophageal ganglion (sp) ; oc, ocelli, 

 anterior and vertical ones, with ocellar nerves leading to them from the "brain ;" 

 oe, oesophagus ; m, mouth ; Ib, labium or under lip ; if, infra-esophageal ganglion, 

 sending three pairs of nerves to the mandibles, maxillae, and labium respectively 

 (not clearly shown in the engraving) ; sm, sympathetic or vagus nerve, starting 

 from a ganglion resting above the oesophagus, and connecting with another ganglion 

 (eg) near the hinder end of the crop; sal, salivary glands (the termination of 

 the salivary duct not clearly shown by the engraver) ; nv, nervous cord and gan- 

 glia; ov, ovary; ur, urinary tubes (cut off, leaving the stumps); ovt, oviduct; 

 sb, sebaceous gland ; be, bursa copulatrix ; ovt', site of opening of the oviduct (the 

 left oviduct cut away) ; 1-10, abdominal segments. All other organs labeled in 

 full. (Drawn from his original dissections by Mr. Edward Burgess). (From 

 Packard's "Zoology," Henry Holt & Co., Publishers). 



is called ecdysis ( ), or moult (Fig. 227), and the 



periods between moults are called "instars." 



It will be remembered that we spoke of a double-life in the frog, 

 not only as applied to its living in water and on land, but as to its be- 

 ginning life looking very much different from what it does as an adult. 

 Practically all insects go through a metamorphosis ( ) 



of some sort, and this is much more complicated than the change under- 

 gone by the frog. 



When insects hatch from eggs (Fig. 241, I, II), and are unlike their 



