408 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 54. 



The tarsi are typically five- jointed with two terminal claws (pi. 64, 

 figs. 1, 10; pi. 65, fig. 1). In the progression of characters we find 

 four-jointed (pi. 69, fig. 1), three-jointed (pi. 75, figs. 1, 7), and 

 two-jointed tarsi without claws. 



The wings have typically eight primary veins — costa, subeosta, 

 radius, medius, cubitus, and three anal. One or more of these is 

 frequently absent, although the first four are always present. 



The prothorax has a tendency to crowd forward into the head. 



The metathoracic praescutum rises from a depressed necklike posi- 

 tion (pi. 66, figs. 1, 5, 6) to a part of the disk (pi. 64, figs. 1, 10) 

 and tends to crowd backward (pi. 69, fig. 1), separating the scutum 

 into two lateral pieces and pushing the scutellum far back (pi. 76, 

 fig. 1). The scutum, in addition to dividing on each side of scu- 

 tellum (pi. 64, fig. 10), also tends to divide transversely from the 

 base of the wing to the scutellum to form the parascutellum (pi. 6u, 

 fig. 8). The postlumbium, although normallj^ intersegmental (pi. 69, 

 fig. 2), has at least in one case become a chitinous part. The pleural 

 suture frequently fails to reach the coxae (pi. 65, fig. 7). The scu- 

 tellum proper never reaches the base of the wings, but a small, abso- 

 lutely detached part is connected by a long axillary cord to the wing 

 (pi. 65, figs. 6, 7; pi. 66, fig. 7; pi. 70, fig. 4; pi. 72, fig. 1). 



Normally the abdomen has the ninth or genital segment ventrally 

 greatly surpassing the tenth or anal segment, and all the other seg- 

 ments normal ringlike. 



In the Halictophagoidea the eighth segment ventrally also is often 

 greatly produced (pi. 75, fig. 4; pi. 78, fig. 3). 



COMPAEATIVE MORPHOLOGY. 



Eyes. — The head is characterized by the large raspberrylike eyes 

 with separated ommatidia. These vary slightly in shape, but are 

 usually spherical and very hairy on the partitions. The number of 

 ommatidia is quite variable (see the various plates). 



Mouth parts. — The mouth parts are extremely simple, being merely 

 a pair of mandibles and a pair of maxillae placed distant from the 

 exposed pharyngeal opening. Labrum and labium are absent in the 

 adult, although present in the last larva. The mandibles in the 

 earlier groups are all elongate, falciform, glabrous, and chitinous 

 (pi. 66, figs. 4, 7). In the Halictophagoidea they are often mere 

 fieshy pubescent appendages. The mandible of Triozocera tnexicana 

 is the most minute yet seen, being reduced to the size of a seta (pi. 65, 

 fig. 4) . The maxillae are fleshy, pubescent appendages usually with 

 a one- jointed palpus (pi. 74, fig. 2), but in Craivfordia with a two- 

 jointed palpus. The palpus is usually terminal, but in Triozocera 

 mexicana (pi. 65, fig. 5) and several species of Xenos it is lateral. 

 In Ltburnelenchus hoehelei a chitinous filament is attached to the 

 basal joint of the maxilla. 



