MECHANICAL PROTECTION AND POISONS 



379 



of the enlarged head force the poison out into the wounds, causing much 

 pain in a man, and death to some smaller animals. 



The insects have developed formidable weap- 

 ons by which they kill their prey and injure their 

 larger enemies. The best known examples are the 

 bees and mosquitoes. The apparatus in the butcher 

 bee (or ground hornet), Scolia dubia, consists of a 

 cuticular formation, the sting, which is made up of 

 several parts and worked by special muscles and 

 nerves, and a poison gland with its reservoir, which 

 is an invaginated portion of an internal integu- 

 ment, if the lower part of the intestine can be so 

 called. As the poison gland is, perhaps, the more 

 important from our point of view, we shall devote 

 our time to that tissue, merely remarking that the 

 several parts of the sting and its sheath are elon- 

 gated processes of chitin formed by long, hypoder- 

 mal cells. 



The duct and reservoir are lined by a very 

 delicate hypodermis, covered by a thin and flexible 

 chitin. The real poison gland is a tubular invagi- 

 nation lined with a cuticle whose walls are invagi- 

 nated into many fine tubes. These tubes extend 

 proximally into a series of elongated columnar 

 gland-cells in whose cytoplasm they pursue a short 

 course and end with a cylindrical enlargement which 

 appears to act as a physiological filter (Fig. 346). 

 The poisonous secretion is conducted by these tubes 

 to the sting, from which it runs in fine streams 

 that emerge behind the barbs. A layer of thin 

 cells with very small nuclei extends between the gland-cells and the 

 cuticle. Their function and origin must be the same as in the odorous 

 gland of Belostoma, which see. 



Among some insects, particularly the larvae of Lepidoptera are found 

 some poisonous spines which are very efficient in their structure and 

 operation. They are specializations of the insect hair, which can be 

 modified to form structures for so many other purposes. 



In the case that we shall select for examination, Sibine stimulea, the 

 hairs are compound structures, extending from several wart-like pro- 

 jections on the sides of the body, and particularly from four large horn- 

 like processes, two of which arise on the head and two on the tail of the 

 larva. The particular hair that we are examining is represented by Figure 

 347, which shows an oblique section of the basal part at the point where 



FIG. 345- Tip of the 

 spine of the sea-urchin 

 jEsthenosoma. p.s., 

 poison sac; /., lance to 

 puncture enemy; mus., 

 muscular cushion to 

 squeeze out poison; c., 

 epithelial cap which is 

 ruptured when the 

 spine is used. (After 

 P. and F. SARASIN.) 



