1 2 Control of the Greenhouse White Fly 



the suitable food. The movement of the larvae is therefore not a migratory- 

 one and when the eggs are laid on an unsuitable plant or the foliage 

 holding them is severed from the plant, they cannot survive. Eggs and 

 feeding larvae on severed foliage shrivel up and die with the drying of 

 the foliage which holds them. 



All the larval stages are distinctly flat after the moult and the growth 

 in each stage is in depth only. The first three become very turgid towards 

 the moult and the skin splits at the junction of the thorax and abdomen 

 by a T-shaped orifice. Through this the larva protrudes the head and 

 thorax and forces itself forward till it can grasp with its legs the leaf 

 in front of the old skin. It then elevates the posterior end of the body 

 thus tearing the old skin away from the leaf. This is heavy through being 

 filled with the "honey dew" and when it is released it falls clear of the 

 leaf as a rule. Walking subsequent to a moult has not been observed 

 but when the scales are crowded a revolving motion is often seen while 

 the larva feels for a clear space. Overlapping of the scales, however, is 

 common in heavy infestations. When the adult emerges from the mature 

 scale the empty shell is left attached to the leaf. 



The fourth stage of the scale (Fig. 4) is always referred to in the 

 literature of the Aleyrodidae as the pupa, but at the beginning of the 

 instar. it is as much a larva as the preceding stages. Its dorsal skin 

 becomes somewhat heavily chitinised and leathery and in its growth this 

 is elevated entire from the leaf, a corrugated palisade of wax forming 

 as the elevation proceeds. The stout waxen case is continued entirely 

 over the ventral surface and the mouth stylets protrude through it. 

 Respiration takes place at folds where the otherwise translucent wax 

 remains opaque white and porous. These breathing folds are the same 

 depth as the palisade and are situated one median posterior and two 

 antero-lateral in the region of the thorax. The dorsal surface carries a 

 marginal fringe of short tooth-like waxen processes arising from bosses. 

 These short spines curl downwards. There is also a system of longer 

 waxen processes standing upright from the scale. Hargreaves mentions 

 and figures eleven pairs of which seven pairs are marginal. In the 

 specimens examined during this work only four pairs could be distinguished 

 from the marginal teeth, viz. one anterior, one over the lateral breathing 

 folds, one posterior to this pair at the level of the first abdominal segment, 

 and one caudal, while in the typical form those on the disc agreed with 

 Hargreaves' description, viz. one pair cephalic, one thoracic, one on 

 the third and one on the folirth abdominal segments. In a few of the 

 specimens mounted from tomato the fifth and sixth abdominal segments 



