METAZOA INSECTA. 491 



structure gives rise apparently to a sessile abdomen, but 

 this condition must not be confused with the true sessile 

 abdomen of the generalized insects, like that of the 

 Orthoptera and Hemiptera. It may be called a pseudo- 

 sessile abdomen which may have arisen from a peduncu- 

 lated abdomen by the reduction of the peduncle and the 

 complete concealment of the posterior part of the meta- 

 thorax by the basal abdominal segment. 



The antennae for some unknown reason have the third 

 joint enlarged. The mouth parts are complex organs 

 and are fitted for piercing, sucking, and lapping. The 

 mandibles (fig. 4, md) and the first pair of maxillae (fig. 

 4, mx ') with their large palpi are modified into sharp 

 piercing organs. The second pair of maxillae (fig. 4, 

 mx ") are composed of leaf-like parts that spread out and 

 are used for lapping sweet fluids. 



The phylogenetic studies of Kellogg 1 have thrown 

 much light on the Dipterous mouth parts. He shows 

 the necessity of first becoming familiar with the mouth 

 organs of generalized Diptera in order to interpret rightly 

 the complex structure of these organs in specialized forms 

 such as the Muscidae (see p. 493). 



The legs of Tabanus are long and the hairs of the 

 cushions of the feet secrete a sticky substance which 

 enables the fly to walk on a ceiling. The process of spe- 

 cialization by reduction has reduced the number of wings 

 from four to two ; hence the name of Diptera, meaning 

 two and wing. These wings are strong and the flight 

 is exceedingly swift. We have already seen that a pair 

 of balancers, also called halteres (fig. 4, ?<:/") represent the 

 second pair of wings. The function of these organs is 

 not known with certainty. They may be sense organs 

 (Sharp) assisting in the perception of sound. They also 

 seem to aid in preserving the equilibrium of the body, 

 and they may be useful in directing the flight of the 

 insect. 



'Psyche, VIII, no. 273, 1899; Biol. Bull., I, no. 2, 1900. 



