19211 Muir—The Phylogeny of llonioptera 117 



in the Thysaiioptera. But here we find two distinct types oi" 

 mouth organs and both are so dissimilar to the Hemiptera that 

 it is not lilvely that one arose from the other. Tlie Psocidse possess 

 a semi-free maxillary style which may represent a condition found 

 in the ancestors of the Hemiptera, but no Psocids that we know 

 today could have been that ancestor because they are far too spe- 

 cialized in certain directions. 



The most generalized head in the ITemi})tera is found among 

 the Heteroptera where the gula is well developed or very large, the 

 head capsule of more normal shape and position, the labium is 

 four-jointed and consists of a submentum, mentum, subgalea with 

 amalgamated paragiossa and lacinia. In some few species the 

 labial palpi are also ])resent. In the Homoptera the gula has 

 disappeared or is represented only by a membrane, and the labium 

 is in close association with the pronotum. The reduction of the 

 gula has drawn the head downward and under, bringing the top 

 of the head to the front and altering the whole shape of the head 

 capsule. In this respect the Cicadoidea are more primitive than 

 the Fulgoroidea. 



In the Sternorhynchi this line of evolution of head set up in 

 the Auchenorhynchi is carried to a still greater extent. In the 

 Psyllidffi the clypeus, frons (bearing the phrayngeal muscles), ten- 

 torial structure and the labium have separated themselves from 

 the head capsule and become still more closely connected with pro- 

 sternum. To accommodate the long mandibular and maxillary 

 setae an invagination at the base of the labium penetrates the 

 prothorax. In the Coccidae the reduction of the head capsule is 

 carried to the extreme, so that the frons, clypeus, tentorial struc- 

 ture, labium and the styles are isolated in a membraneous area 

 and appear to be thoracic structures. The line of evolution of the 

 head of the Hemiptera is so well defined that any student of com- 

 parative morphology, except an extreme mutationist, will agree 

 that the Psyllida? are highly specialized and do not represent the 

 ancestral form. 



The Hemiptera can be divided into two groups on the condition 

 of the alimentary canal. In the Heteroptera, Fulgoroidea and 

 Cicadellidfe this is simple. In the Cicadadse, Cercopidae, Mem- 

 bracidae and the Sternorhynchi the posterior portion of the midgut 

 is in intimate contact with the anterior portion and forms a filter 



