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The walls of this sac are double, and completely separated from the true body of the pupa, except at the points where the budding extremities take their

origin ; and between its walls lie the developing appen- . dages of the adult, which are sometimes cut across more than once, on account of their being folded on themselves, In the middle line below are seen the upper and lower lips, with the hypopharynx between them, and just outside these the mandibular and maxillary styles.

Immediately outside these are seen sections of the palpi, followed by six sections of the three folded pairs of legs; and still further out a couple of sections of the also folded antennw. Uppermost and outermost of all are seen sections of the limp, crumpled wings.

If we now compare with this fig. 2, a section of a full-grown pupa at the same level, we find that the essential difference lies in the fact that the great air Space is entirely wanting, and that the larva is flat- tened from dorsum to venter, instead of being, like the pupa, compressed from side to side. The great masses of muscles are as yet wanting, and represented only by soft nuclear tissue. But apart from this, the two sec- tions are completely comparable, the appendages, as well as the viscera, being already more or less com- pletely represented. Taken as it is, at the level of the origin of the wings, the section divides the intestinal canal exactly at the point of entry into it of the four pairs of hepatic crypts, which are thus here represented by four semilunes.

The wings themselves, already far developed, are on account of their less horny consistency distinctly thicker, if less in area than in the adult. Following the periphery of the section, wé find on either side of its ventral aspect a compartment containing sections of the rudiments of the three legs, and in the middle line a small mass of tissue, as yet not distinctly differen- tiated, but which obviously represents the commence- ment of the unpaired structures of the mouth to be found in the corresponding position in the pupa.

If we now pass to the examination of a section carried across this mature larva at the anterior part of the meso- notum, we find that between the sections of the rudi- mentary legs and the now separate median structure le a pair of developing masses, which probably repre- sent the commencement of the paired mouth organs.

The intestinal canal is lined with a plain endothe- lium of quadrangular cells, and on either side lie the salivary glands.

Coming still further forward to the examination of fig. 4, a section taken at the level of the pronotijum, we find, springing from the dorsal aspect of the section, the already far-developed respiratory horns of the pupa, and outside these the anterior parts of the wings; while the ventral part of the periphery of the section is oceu- pied by sections of the rudiments of the three legs.

The median mass has disappeared, and it is evident from this circumstance that the area of tissue so placed really corresponds, not to the thorax, but to the parts of

the head of the adult insect, placed though it be, to all superficial appearance, under the very middle of the thorax. It is further worthy of notice that the lining of the intestinal canal is no longer glandular but chiti-

THE JOURNAL OF TROPICAL MEDICINE. [June 15, 1903.

nous ; in other words, that the section is in advance of the midgut.

The other structures cut across are the salivary ducts and two large longitudinal air-vessels.

It remains only to examine a section carried across the metanotum, as in fig. 5. In this we find, surrounding the intestinal canal, the four great hepatic crypts of the larva. Springing from the side of the dorsum is — the rudiment of the ballancer, which already contains — an obvious ganglionic process, which is to be afterwards developed into what we believe to be an organ of hear- ing. Next to this we find, cut across, the hinder ends of the wing; and quite ventrally, a mass representing the hindmost pair of legs.

I have sectionised a considerable number of larve, but this is the only imstance in which the specimen selected has proved to represent the larva in its last and most advanced ecdysis; and it is evident that the change from larva to imago is by no means as abrupt as we have been accustomed to think, and is spread not merely over one, but at least two changes of skin.

The position of the masses representing the mouth- parts is at first rather difficult to explain, but is suffi- ciently easily understood when similar sections of larva and pupa are compared, though it opens up a distinct doubt as to whether the structures of the proboscis are really entirely referable to the head, as it seems really rather the more probable that, though attached to and in front of the headin the imago, they may really appertain to the ventral aspect of the thoracic seg- ments; though, of course, the fact of their appearing | in the same transverse section by no means necessarily © implies this, as it is quite possible that the germinal — masses representing these median prolongations of the — adult head may be bent round so as to lie between but — on the same level as truly thoracic structures ; and it is needless to remark that this explanation is the easier to believe, even though it be the less easy to understand, when one is confronted with actual sections.
















EXPLANATION OF ILLUSTRATION.

Fic. 1.—Transverse section of thorax of pupa, at the level of — the origin of the wing (for comparison with the other sections). — Fia. 2.—Transverse section of a mature larva at the same — level. Fie. 3.—Transverse section of thorax of mature larva some-_ what in front of preceding, showing the body of the salivary — gland and rudiments of proboscis and limbs. =| Fic. 4.—Transverse section of thorax of mature larva at the level of the origin of the respiratory syphons, in front of body of salivary gland, the duct of which now appears. ‘ Fic. 5.—Transverse section of thorax of mature larva at the — level of the origin of the ballancers. Mi In the above figures—a, thoracic muscles; 6,. ballancer; d, dorsal vessel; e, cesophagus; /, air cavity or float between body and appendages of pupa: /, hepatic tubules; k, hypo- pharynx; /, legs; lbr, labrum; lm, labium; mb, mandible; mx, maxilla; 2, ventral nerve cord; p, rudiment of pneumatic vesicle; pr, proboscis; s, salivary gland or duct; ¢, main trachea ; w, wings. «

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