184 DURHAM DIPTERA. 



XXVIII. 279. 



25 (24). Smaller species (8 mm.). The dust spots on the side 



of the frons grey and ill-defined. 



(1488. PL zmmarginatus, Ztt.) 



26 (23). Darker species, with usually conspicuous blackish 



markings on the hind femora and tibiae, but this 

 character is not a very safe one. 



27 (28). Larger species (8 mm.). The dust spots on the side 



of the frons moderately large. 



PL clypeatus, Mg. (1491) 



28 (27). Smaller species (7^ mm.). The dust spots on the 



side of the frons small. PL angtistatus, Ztt. (1492) 



29 (16). Hind legs entirely orange, without the faintest trace 



of any dark markings on at least the hind femora. 



30 (31). Side of the frons with fairly well defined golden dust 



spots. (1489. PL fulviventris, Mcq.) 



31 (30). Side of the frons with undefined greyish dust spots. 



(1490. PL fierfiallidus, Verr.) 



32 (11). Antennas partly yellow, at least at the base of the 



third joint beneath. 



33 (36). Abdominal spots bluish. 



84 (35)^ Abdominal markings resolved into spots. 



PL albimanus, Mg. (1485) 



35 (34). Abdominal markings forming cross bands, the second 



and third pair of spots meeting broadly in the 

 centre. (1496. Melanostoma ambzguum, Fin.) 



36 (33). Abdominal spots yellowish. 



37 (42). Abdominal spots tending to being quadrate (VII. 11). 



38 (39). Size 9J mm. Abdomen with four pairs of sharply 



defined pinkish-yellow subquadrate spots, the pair 

 on the fifth segment being very distinct ; face con- 

 siderably produced at the mouth. 



PL peltatus, Mg. (1483) 



39 (38). Size 6^—8 mm. Only three pairs of abdominal spots 



distinct, the pair on the fifth segment being in- 

 distinct or absent. Face rather fiat. 



