190 THE ENTOMOLOGIST. 



the water, and had the tracheal sj'stem feebly or not at all de- 

 veloped ; absorbing oxygen chiefly through the skin and tending, 

 perhaps, as much in the direction of an aquatic as a terrestrial 

 life. It had no distinct metamorphosis, was oviparous, bisexed, 

 changing little in appearance from the time it emerged from the 

 egg until it was adult and capable of reproduction. The mouth 

 structures were generalized, feebly developed ; but with at least 

 three, and possibly four, pairs of composite structures corre- 

 sponding to mandibles, maxillre and labium of our existing 

 insects. The possible fourth pair may have been an endo-labium 

 and, perhaps, the labrum with its attached epipharynx may have 

 required a fifth pair of structures. Most essential of all was an 

 inherent power of variation and adaptation, and probably, as 

 with some of our present Thysanurans, reproduction was rapid 

 and enormous numbers existed. The first important differentia- 

 tion occurred in the mouth structure long before wings became 

 developed, tending on the one hand to a perfection of all or most 

 of the parts, or to a mandibulate type ; on the other to a loss of 

 certain of the structures, accompanied by a different development 

 of the others, forming a haustellate type. In this latter branch 

 the mandibles were never developed, the maxillary structures 

 became elongated, separated into their parallel parts, and the 

 labium became obsolete as a functional organ. Just how many 

 intervening orders existed between Thysanura emandibulata and 

 the best development of the haustellate structures it is im- 

 possible to say ; but the only one in existence at the present 

 time is Thysanoptera, also called Physopoda, otherwise Thrips. 



This order I consider a distinct one on the same branch from 

 which arose the Hemiptera, but forming merely a short spur 

 and retaining characters which were soon lost in the main and 

 more vigorous branch. It is a survival which has lost the power 

 of further development, and can do no more than merely main- 

 tain itself. The main branch formed the Plemiptera, or, as I 

 prefer to call them, the Ehyngota of to-day ; the mandibulate 

 parts being completely lost, the labium losing all external ap- 

 pendages, and the maxillae forming the jointed beak with its 

 enclosed lancets. 



The Thysanoptera and Ehyngota of all the existing orders 

 are the only ones that do not have functional mandibles in some 

 stage of their development. They are haustellate from their 

 birth, and the character of the mouth parts never changes. In 

 all the other orders, either larva; or adults, or both, are man- 

 dibulate. I am aware that there are seeming exceptions in 

 several orders, notably the Diptera ; but it will hardly be dis- 

 puted that this order is of a mandibulate stock, and many larvae 

 have the parts well developed. 



It results from the views just stated that the Thysanoptera 

 and Ehyngota are a division equal in value to all the other, or 



