Fig. 158.—Larva of Musca vomitoria (Calliphora vomitoria); below: of natural size; above, enlarged. (Leuckart.) (From Tyson.)
Fig. 159.—Larva of Anthomyia canicularis, enlarged. Rarely found in the stool. (Gould.) (From Tyson.)
The larvae usually obtain access to the alimentary tract in food taken in by the mouth. Flies of the genus Sarcophaga are prone to deposit their larvae on food, especially meat that is somewhat tainted. Other flies, as Musca or Anthomyia, may lay their eggs on food. Flies of the genus Anthomyia tend to lay their eggs on plants.
It is possible for a fly to deposit its eggs or larvae about the anus while the man is at stool.
Great care must always be observed to assure one’s self that fly larvae, which may be present in the stool, have not originated from larvae deposited on the stool subsequent to its passage.
Determination of Dipterous Larvae
There are certain points in the anatomy of dipterous larvae which must be considered in recognition of the genus or family of the flies concerned in the various myiases. The broad extremity is the posterior one and the tapering one the anterior. The dark hook-like processes, which may be in pairs or fused, project from the anterior or head end and above them is a pair of projecting papillae. The second segment from the head has on either side projecting hand or fan-like structures with varying numbers of terminal divisions, 4 to 40 or more. These are the anterior spiracles.
The large terminal segment has on its posterior surface two chitinized plates with 3 slits of various architecture in each. These are the posterior stigmal plates and are the structures we pay particular attention to in identification. In the early larval stages there is only one slit; in the second stage there are two. It is only in the fully developed larval stage that we note the characteristic 3 slit stigmal plates. The presence or absence of a rounded protuberance or button at the base of each stigmal plate should be looked for. The area carrying the stigmal plates may be sunken to form a pit. (See Fig. 135.)
Key To Larvae of the Myiases. (Banks.)
| 1. | Body with lateral and dorsal spinose processes | Homalomyia. |
| Body without such processes | 2 | |
| 2. | Body ending in two fleshy processes; rather small species | 3 |
| Body truncate or broadly rounded at end | 4 | |
| 3. | Processes bearing the stigmal plates; body about 5 mm. long | Drosophila. |
| Processes not bearing the stigmal plates; body 10 mm. or longer | Piophila. | |
| 4. | But one great hook; posterior stigmal plates with winding slits; no distinct lateral fusiform areas; tip of body with few if any conical processes | Muscinae. |
| With two great hooks; slits in the stigmal plate not sinuous | 5 | |
| 5. | No tubercles about anal area; no distinct processes around stigmal field | 6 |
| Distinct tubercles above anal area; often processes around stigmal field; lateral fusiform areas usually distinct | 7 | |
| 6. | Stigmal plates on black tubercles; lateral fusiform areas distinct | Ortalidae. |
| Stigmal plates barely if at all elevated; lateral fusiform areas indistinct; stigmal plates often contiguous or nearly so; slits long and subparallel | Trypetidae. | |
| 7. | Slits in stigmal plates rather short, and arranged radiately | 8 |
| Slits slender and subparallel to each other | 9 | |
| 8. | Two tubercles above anal area; stigmal field with distinct processes around it | Anthomviidae. |
| Four or more tubercles above anal area; slits of stigmal plates usually pointed at one end | Muscinae. | |
| 9. | A button to each stigmal plate; slits rather transverse to body | Calliphorinae. |
| No button to stigmal plates, slits of one plate subparallel to those in opposite plate; plates at bottom of a pit | Sarcophagidae. |