192 THE ENTOMOLOGIST. 



closely joined to the mesothorax at the base, and is not movable, 

 while the tendency is for the head to become free from it, and at 

 all events not to be inserted into the thoracic segment. While 

 we do not have anywhere in this series a distinct neck, yet on 

 the other hand there is nowhere a retraction of the head into 

 the prothorax. In this series both pairs of wings are similarly 

 developed, both as to size and as to general character, while the 

 secondaries, though frequently covered by the primaries, are 

 never folded beneath them in any way. The primaries are 

 always functional. 



The lowest in this series, and almost the simplest in general 

 structure, are the Isoptera, where all the thoracic segments are 

 well developed, and the prothorax is scarcely dominant, though 

 larger and almost free from the others. The wings are very 

 much alike, the secondaries only a little larger than the pri- 

 maries, and both are laid flat upon the abdomen. The mouth 

 structures are almost identical with those of the earwigs and 

 some of the Orthoptera. I believe the members of this order 

 are amongst the most primitive of all the terrestrial winged 

 insects now existing, and among the most ancient, though 

 remarkably specialized in certain directions at the present time. 

 Though at first glance it would seem as if these insects should 

 belong to the series in which the prothorax is free, yet the 

 character of the wing structure forbids this association, and 

 makes the Isoptera a natural stem from which were derived the 

 Mallophaga, Corrodentia, and Neuroptera. 



[The phylogeny of the orders included in the second division 

 is then considered.! 



The third series, in which the prothorax becomes much 

 reduced in size and firmly articulated to the mesothorax, has 

 the body parts as a whole much more closely jointed and 

 globular. The tendency is to bring the origin of the legs close 

 together, and to the loss of the sternum as a distinct part or 

 sclerite between the coxas. The mesothorax becomes dominant 

 and best developed, bearing also the chief organs of flight. As 

 a whole, subject to many exceptions, the tendency is to the 

 development of the primaries, which are never reduced to mere 

 wing-covers and never lose function. The tendency seems to be 

 rather to a decrease in the size of the secondaries, as in Hymen- 

 optera, and to their total loss, as in the Diptera. There is, 

 however, a great deal of variation in this respect, and the most 

 that can be justly said is that in this series the secondaries never 

 become the only, or primary, organs of flight. Another point of 

 very great importance is that here the head is nearly always 

 more or less free or well separated, tending to the formation of a 

 distinct neck ; while there is never any insertion of the head 

 into the prothorax. This fact will become very striking when 

 the orders that are placed here are compared with those in the 



