THE CITRUS THRIPS. 13 



dividuals occur. The eyes are dull red to dark purple. The average 

 length varies from 0.45 mm. (about 0.017 inch) at the beginning of 

 the instar to 0.90 mm. (about 0.035 inch) just before molting to the 

 propupa. 



FEEDING HABITS. 



The larva becomes active immediately after emergence, moving 

 about over the leaf or stem, swinging the antennae up and down, and 

 surveying its surroundings for a place to feed. When feeding, the 

 citrus thrips does not insert the mouth-cone itself into the plant 

 tissues, and thus does not cause holes and raggedness in the leaves, 

 as the pear thrips does, for example. The feeding puncture of the 

 citrus thrips is made by bringing the tip of the cone into contact with 

 the surface of the plant and moving the head quickly downward and 

 slightly backward. This movement results in a slight folding, or 

 elbowing of the cone at a point near its juncture with the head and 

 in some way exerts a leverage upon the lancets, which are thus pushed 

 into the tissue, starting the flow of cell sap, which is sucked up 

 through the tubular interior of the cone.. 



DURATION OF LARVAL FEEDING STAGE. 

 \ 



The first of the two instars or stages of growth of the larva ranged 

 from a minimum of 1 day to a maximum of 13 days, with an average 

 of 3.7 days, in specimens in which the actual casting of the skin 

 was observed. The second instar of these specimens ranged in dura- 

 tion from 2 to 9 days, with an average of 4 days. An abnormal 

 maximum of 29 days was readied by one specimen in November, the 

 insect ultimately dying from exposure to prolonged low temperatures, 

 without changing to the propupa. The two larval instars may be 

 said to be of about equal duration. 



The duration of the complete or feeding stage of the larva ranged 

 from 3 to 21 days, the average for the entire period from March 25 

 to November 5 being 7.8 days. Great individual variations in the 

 length of the immature stages are to be expected and do not have 

 any particular significance in a practical way, but a knowledge of the 

 average variation in different periods of the season and in different 

 seasons, due largely, as they are, to climatic factors, are of value in pre- 

 dicting what the severity of a given infestation will be. The principal 

 seasonal variations in the duration of the larval stage, with a record 

 of the prevailing temperature, which is the principal factor affecting 

 the length of the stage, are given in Table IV. It will be seen that 

 in seasons such as 1911 the larval stage will range from about 4| 

 to 7 or 8 days in the period from about the middle of May to the 

 middle of September, and in the cooler weather of March and April 



