16 BULLETIN 616, U. S. DEPARTMENT OF AGRICULTURE. 



of predatory insects than any other stage, and therefore pupation 

 occurs in places practically inaccessible to any but the smallest in- 

 sects and mites. The pupae are scattered widely, thus limiting the 

 possibility that any great number will be destroyed at one stroke. 

 They have been found in the following situations: On the ground 

 under the trees; in curled, dried leaves, and under a mat of fine 

 cobweb and dust on dry leaves; in the split tips of leaf stems; in 

 small crevices in the bark of dry twigs. In clean-cultivated young 

 orchards, where but few dead leaves and twigs collect, the thrips 

 pupate in crevices near the base of the trunks, under dead and living- 

 fluted scales (I eery a purckasi Mask.), and in similar obscure situa- 

 tions. In only two instances was pupation found to have occurred on 

 the upper part of the tree : One in which a propupa was found in a 

 distorted leaf, and another in which one specimen occurred under the 

 cocoon of a caterpillar on an orange leaf. To find pupae of the citrus 

 thrips requires a minute and painstaking search. If no better shelter 

 offers, the insect gathers particles of dust, wood, leaves, etc., about 

 itself, effectively concealing it. In captivity a large proportion of the 

 pupating specimens crawled into the cotton plugs of the rearing 

 bottles and under the split bark at the ends of orange stems used 

 as food. No food is taken during the pupal stage. 



DURATION OF THE PUPAL STAGE. 



The duration of the pupal instars may be accurately determined, 

 as once a spot has been chosen for pupation the insect seldom moves 

 from it, and cast skins of the last larval and both pupal transforma- 

 tions generally remain nearby. The first pupal instar ranged from 

 1 to 9 days, with an average for the entire active season, from April 1 

 to November 8, of approximately 2 days. The second instar ranged 

 from 1 to 25 days, with an average of 5.5 days for the period from 

 April 4 to December 2. The 25-day maximum period is of course 

 exceptional, and would occur in comparatively few individuals of 

 very late hatch. 



The entire variation in duration of the complete pupal stage in 

 the period from April 1 to December 2 was from 2 to 28 days, with 

 an average of 7.5 days. The important general variations in the 

 length of the pupal stage in different periods of the season are 

 shown in Table V. It will be seen that growth of the pupa, as well 

 as of the egg and the larva, is slow in the cool weather of spring and 

 fall, when the temperature ranges between 50° and 60° F., and 

 that it is considerably accelerated in midsummer, with temperatures 

 above 60° F. 



