PHYLUM ARTHROPOD A 243 



before the wings reach their functional form have led to the 

 formulation of a hypothetical plan of primitive venation (Fig. 

 118) from which all wing patterns are derivable, though all of the 

 homologies are not readily observable by the beginner in this 

 study. 



Plan of Wing Venation. — In the plan of primitive venation 

 (Fig. 118), eight principal veins are present and most of these may 

 be branched. Beginning with the anterior margin when the 

 wings are at right angles to the body, these veins are: costa, 

 subcosta, radius, media, cubitus, first anal, second anal, and third 

 anal. Changes from the hypothetical type occur either by the 

 addition of new veins or branches (Fig. 120) or through the 

 reduction in the number of veins (Fig. 119) through atrophy or 

 through coalescence of two or more veins. Cross-veins fre- 

 quently connect two of the longitudinal veins. 



The Abdomen and Its Appendages. — The abdomen bears a 

 highly variable number of segments in the various groups of 

 insects. A study of insect embryology shows that the abdomen 

 consists normally of eleven segments, but in later development 

 adjacent segments may coalesce and in some adults they are 

 telescoped one with another. The wall of each segment is 

 composed of a tergum and a sternum united by a pair of pleural 

 membranes. The segments are typically without appendages 

 and are approximately similar except near the caudal extremity 

 where certain segments are more or less modified. In the Thy- 

 sanura, rudimentary abdominal limbs occur and in the embryos of 

 some other insects each segment may bear a pair of rudimentary 

 appendages. Those of the first seven abdominal segments are 

 usually lost during early embryonic life, while the last two 

 or three pairs frequently persist to form the genitalia — the 

 genital claspers of the males and the ovipositors of the females 

 (Fig. 121). 



A true ovipositor consists of three pairs of valves, called gono- 

 pophyses, arranged as a dorsal and a ventral pair surrounding an 

 inner pair. The inner valves form a channel through which 

 the eggs are conveyed. There is strong evidence that these 

 three pairs of gonopophyses represent the paired appendages 

 of the eighth, ninth, and tenth abdominal somites and are seri- 

 ally homologous with the thoracic legs. In the Hymenoptera, 

 the ovipositor is modified as a stinging organ and has poison 

 glands associated with it. 



