302 FOREST ENTOMOLOGY. 



The abdomen should be carefully noted, both as fresh or prepared 

 specimens, as the genital segments are so clearly defined that the 

 sexes are very easily determined, more especially if a side view be 

 presented. 



The development of the creature, from its early larval stages to the 

 winged insect, forms a most interesting study, and should also be 

 examined both as fresh and as prepared specimens. The former will 

 show the wax cells, the latter the structural features. Witlaczil 

 describes four distinct larval stages from embryo to imago, and the 

 student will be well repaid by following up the development. 



As regards classification, it may be noted that Dr Franz Low, in 

 his paper " Zur Systematik der Psylloden, 1878," divides the family 

 into four sub-families — viz., Liviinse, Aphalarina?, Psyllinae, and 

 Triozinge. So far, however, as forest insects are concerned, the sub- 

 family Psylliuce are the most important. 



The Psyllina?. are again divided into two genera — viz., Psyllopsis 

 and Psylla. In the former genus we have two species on ash-trees, 

 and in the latter there are several species found on different trees. 



Psyllopsis praxinicola (Fiirst). 



If the foliage of our large ash-trees be beaten over an inverted 

 umbrella during the summer months, we shall find a very large num- 

 ber of green Psyllidae, amongst other insects. These are Psyllopsis 

 fraxinicola. They cannot be said to be injurious to the foliage to 

 any appreciable extent, inasmuch as they simply suck the juices, and 

 do not deform the leaves in any way whatever. 



The whole body of the insect is of a pale-yellow colour, the tips of 

 the antennae and the claws of the feet being often darkish-brown. 

 Length 3 mm. 



Psyllopsis fraxini (Linn.) 



In early summer the foliage of the young ash-trees in the nursery- 

 rows is often rolled up, as shown in fig. 284, the main char- 

 acteristic feature of the damage being that the whole of the com- 

 pound leaf is injured. Sometimes a single compound leaf springing 

 from the stem may only be affected, and at other times the damage is 

 so bad that almost the whole foliage is injured. As this injury appears 

 most conspicuous just when the plant is most active, it is obvious that 

 the growth must be appreciably affected. A careful examination will 



