12 BULLETIN" 173, U. S. DEPARTMENT OF AGRICULTURE. 



The mouthparts of the Thysanoptera present many difficulties for 

 study and are not thoroughly understood. They are so modified 

 that various writers have disagreed regarding their homologies. 

 They appear, however, to belong chiefly to the suctorial type, and 

 they show many traces of a transition from the mandibulate type to 

 the suctorial. (See PL I, fig. 7.) Viewed as a whole, the mouthparts 

 appear as a broad and jointed cone attached to the posterior edge of 

 the underside of the head and resting for a large part under the 

 pronotum. The apex of the cone is quite sharp, but not so slender 

 and drawn out as in the Hemiptera. The mouthparts as a whole 

 are strikingly unsvmnietrical. The most, evident marks of this are 

 the forms of the labrum and the left mandible. The first, which 

 makes the front wall of the cone, is unsymmetrical in the whole 

 order, but especially so in the Terebrantia . It is irregularly triangular 

 hi form and is attached by its broad base to the clypeus. It becomes 

 narrower as it approaches the tip and is usually rounded in the 

 Terebrantia but more 1 variable in the Tubulifera, where it is pointed 

 in some species and broadly rounded in others. The maxillae are 

 broad and flat, and constitute the side walls of the mouth cone. They 

 also taper toward their tips. The labium forms a hind wall of the 

 mouth cone and is usually considerably broader at the tip than at 

 the other part-. Within this hollow cone lie che piercing organs, 

 which are three in number. First, there is a single large mandible 

 lying on the left side of the mouth cavity, whereas the right side has 

 no corresponding member. 1 The other two organs arc the maxillary 

 lobes. These are more slender and longer than the mandibles and are 

 developed alike on each side. All of the mouthparts are strongly 

 chitinized at the tip. being more so hi the adults than in the ^a T se 

 although the mouthparts of the latter are otherwise closely similar 

 to the former. 



The members of this order are thought to use the mandibles for 

 piercing the exterior portion of the plants, while the maxillary lobes, 

 which are longer, are used to penetrate deeper into the tissues, and 

 are moved with a rasping motion, causing the juices of the plant to 

 flow, so that they may be sucked up into the alimentary canal. In 

 feeding, as observed by aid of a hand lens, both adults and larvae 

 exhibit an up-and-down motion of the head combined with a forward 

 motion which might be properly termed rooting. Most of the species 

 under the writers' observation prefer to enlarge a wound into the 

 plant tissues where the juices flow more readily rather than to select 

 new areas for feeding. This continual macerating of the fruit by 

 the pear thrips for a period of several days causes on deciduous 

 fruits what is known as the characteristic pear-thrips scab, which 



1 The mandible in the Tubulifera is shorter and more bent than in the Terebrantia. 



