388 



The tracheoles are structurally quite different (figs. 76, 

 81) ; each group or branching system of tracheoles is essenti- 

 ally a unicellular structure, of remarkable dimensions. It 

 consists of a large clear tube, which soon branches into two; 

 from these branches numerous smaller trunks come off, and 

 these ramify amongst the organs of the larva. The tubes are 

 entirely devoid of a chitinous intima, spiral or plain, and 

 never, so far as I could observe, terminate within the cells of 

 any tissue. The nuclei are oval, and very large, measuring 

 at the end of the first instar 21/x in length, 8/x in breadth; 

 nucleoli are absent; the chromatin is scattered throughout 

 the nuclear space, but two karyosomes are generally present 

 (fig. 10). The nucleus is usually situated at, or a little 

 beyond, the first point of branching. 



The tracheoles of the imago, which will be referred to 

 later, differ somewhat from these larval tracheoles; neverthe- 

 less, there is a close similarity between the two, and the 

 development of the latter may be inferred from what is 

 observed to occur in the development of the former. 



There they are formed invariably as outgrowths from the 

 tracheal trunks, and there can be no doubt that it is by this 

 method that the larval tracheoles are formed during embryonic 

 life. They are to be looked upon as modified tracheal 

 epithelium cells, which grew in size and developed into cells 

 to which the name Giant Tracheoloh lasts may be applied. 

 These cells then began, still during embryonic life, to grow 

 out from the tracheae and developed the tracheoles from 

 themselves, as they grew out. It is difficult to determine 

 exactly how this happened ; probably the great tracheoloblast 

 which already enclosed, on account of its size, a considerable 

 portion of the lumen of the trachea, began to grow outwards 

 at one end. As it grew outwards its two free edges fused 

 together, forming a tube. A tracheoloblast in this condition 

 leaving the renovated longitudinal vessels and growing out- 

 wards to form a tracheole of the imago is shown in fig. 82. 

 The cell with its great nucleus then grew further and further 

 out, secreting the main branch of the system after it, as it 

 advanced. Soon, however, the nucleus ceased to advance, 

 perhaps on account of the pressure of the fat body, which 

 occupies so much of the haemocoele, and the ramification of 

 the tubes began by a different method ; protoplasmic out- 

 growths were produced from the termination of the tracheolo- 

 blast, probably by the frequent division, within the main 

 substance of the cell, of its lumen. The probable method of 

 branch formation within the main portion of the tracheolo- 

 blast is shown diagrammatically in text figures A-F. Several 

 pairs of these systems of tracheoles occur in a single segment. 



