Morphology and Life Cycle

Because of the large number of myiasis-causing flies, there is a diverse range of characteristics regarding morphology and life cycle. This website will examine in detail the morphology and life cycle of two of the most clinically and economically relevant insects: Dematobia hominis (human botfly) and screwworms (Cochliomyia hominivorax and Chrysomya bezziana). When appropriate, important properties that are common to all myiasis-causing flies will be highlighted.

For more specific information on reservoirs, vectors, and transmission to the host, click the appropriate link in the navigation panel.


Dermatobia hominis 5,6,8,9

Image source: http://icb.usp.br/~marcelcp/Imagens/musc32.jpg

· Egg hatching within the host is triggered by changes in the ambient temperature (i.e. encountering a warm-blooded organism), and larvae can penetrate host skin in under 10 minutes. Like most other myiasis-producing flies, the botfly larvae undergo three stages of development known as instars. During development, the larvae migrate deeper within the host tissue. Incubation time within the host is anywhere from 5 to 12 weeks. After the conclusion of the third instar, the larvae will emerge from the host, drop to the ground, and begin pupa formation. Under warm and human conditions, an adult botfly will emerge after 2 weeks and have a life expectancy of 9 to 12 days.

Image source: http://medent.usyd.edu.au/fact/myiasis.html

· The image above is of a third instar D. hominis larva (bar = 1 mm). The rows of spines are used for boring through the host’s tissues as well as for attachment. Mouth hooks are also present.


Screwworms 5,10,11

 

. Image source: http://www.fao.org/docrep/003/t0756e/T0756E04.htm

Image source: http://usembassy.or.cr/screwwrm.html

· The complete life cycle of the screwworm is about 21 days. An adult screwworm is about twice the size of a common housefly. See the section below for information about maggot morphology (Cochliomyia hominivorax entry).


Additional Notes: 5,12

The incubation time for parasitic fly larvae can vary widely. D. hominis has one of the longest known incubation times. Most other parasitic larvae have an incubation time in the host of about 1 to 2 weeks. For example, larvae of the Tumbu fly (Cordylobia anthropophaga) mature and leave the host in about 10-12 days. They hatch from eggs within 2 or 3 days after entering the host.

The following table, adapted from the Department of Medical Entomology at the University of Sydney (http://medent.usyd.edu.au/fact/myiasis.html), briefly highlights the diversity of maggot morphology even among just a few species of flies.

Species of diptera

Incubation period

Appearance of mature maggot

Cochliomyia hominivorax

4-8 days

Typical maggot shape, 15-17mm long, bands of spines encircling anterior margin of each body segment.

Cordylobia anthropophaga

10-12 days

Oval, 11-15mm, 3 curved slits in spiracles, numerous small black spines.

Cordylobia rodhaini

10-12 days

Up to 23mm long, scattered spines, 3 sinuous slits in each posterior spiracle.

Dermatobia hominis

5-12 weeks

18-25mm long, pair of flower like anterior spiracles, spines in rows.