Larval Development ELMO for parts of a larva One larva (maggot) is produced from each egg. The body is tapered from the anterior (front) end to the posterior (back) end: – Anterior end has mouth and hooks – Posterior end has breathing apparatus (spiracles) Entomologists can distinguish different species by the shape of the spiracles, as well as body size and shape.
Further Development Larvae feed off the corpse. After 3 rd instar stage, larvae crawl away from the corpse and burrow into nearby soil to transform into the pupal stage. Larva emerge from pupal case as an adult fly.
Species Identification Blow fly House fly Flesh fly
Larval Identification Based primarily on spiracular openings Maggots have 2 sets of openings for respiration: 1.One set is on the lateral side of the 3 rd body segment. 2.One set is on the posterior end. (This is the one mainly used for species identification.)
Larval Identification, continued…. Refer to your drawings of larvae. 2 dark circles on the posterior end are called spiracular plates. Spiracular plates contain the spiracles (openings) for breathing. Pattern of spiracular plates and openings are unique to each species. The dark ring surrounding the spiracular plate is called the ____________, and it regulates the opening of the spiracles. The peritreme may be a complete circle or slightly incomplete.
Larval Identification, continued…. Within each peritreme may be 0-3 spiracular slits (openings), which may be straight or S- shaped: – 1 st instar = 0-1 openings – 2 nd instar = 2 openings – 3 rd instar = ???
Life Stage Identification Most flies deposit eggs, but Sarcophagidae (flesh flies) lay living larvae. Eggs incubate a few hours to several days. 1 st instar larva stage lasts less than 36 hours. 2 nd instar stage usually lasts the same amount of time as 1 st instar. 3rd instar stage lasts longest. Maggots get larger with each stage, but the posterior spiracles change each time. (HOW?) What happens after 3 rd instar stage?
Thermal History Logically thinking, how would you expect temperature at the scene to affect the life stages of insects? Once you have identified the species and instar stage, you can research the literature to find how long it takes the insect to reach that stage at the scene’s average temperature.
Thermal History, continued…. Example: You have 3 rd instar maggots of Phormia regina, and the mean temperature from the scene was 22 °C. You find that P. regina takes 70 hours to reach 3 rd instar at that temperature. Using the “accumulated degree hour” method, 70 hours x 22 °C = 1540 accumulated degree hours (ADH)
Thermal History, continued…. We will use the daily maximum and minimum temperatures to determine average daily temperature, so we will have to subtract 6 °C from our average. ELMO for example of Cochliomyia macellaria. Demonstrate use of dichotomous key for tomorrow’s lab.