ANATOMY AND PHYSIOLOGY 



43 



tral duct," or also into the "side ducts," which are specially fitted to 

 convey quantities of fluid too small for the main tube. For a detailed 

 account of the highly complex and exquisitely adapted mouth parts of 

 the honey bee, the reader is referred to Cheshire's admirable work, 

 Packard's Text-Book, or Snodgrass' The Anatomy of the Honey Bee. 



Segmentation of the Head. The determination of the number of 

 segments entering into the composition of the insect head has been a 

 difficult problem. As no segment bears more than one pair of primary 

 appendages, there are at least as many segments in the head as there are 



8 



FIG. 57. Paramedian section of an embryo of the collembolan Anurida tnaritima; to 

 show the primitive cephalic ganglia, i, ocular neuromere; 2, antennal; 3, intercalary; 

 4, mandibular; 5, superlingual; 6, maxillary; 7, labial; 8, prothoracic; 9, mesothoracic; a; 

 antenna; /, labmm ; /*,labium ; I 1 , I 2 , I 3 , thoracic legs; m, mandible; mx, maxilla. After 

 FOLSOM. 



pairs of primary appendages. On this basis, then, the antennae, man- 

 dibles, maxillae and labium may be taken to indicate so many segments; 

 but in order to decide whether the eyes, labrum and hypopharynx repre- 

 sent segments, other than purely anatomical evidence is necessary. The 

 key to the subject is furnished by embryology. At an early stage of 

 development the future segments are marked off by transverse grooves 

 on the ventral surface of the embryo, and the pairs of segmental appen- 

 dages are all alike (Fig. 197), or equivalent, though later they differen- 

 tiate into antennae, mouth parts, legs, etc. Moreover, trie nervous 



