Selective pressures on carrions and larval development strategies 

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Necromass and Necrobiome

Necromass is dead biomass and is an important habitat for many different organisms (Benbow et al., 2019). The present work will concentrate on vertebrate carrions, which include human corpses, and put aside ecosystems of dung, decaying leaves or wood. The necrobiome defines the community of saprophytic species (i.e., those that are living on dead organic matter) that are involved in the decomposition process of necromass (Benbow et al., 2013; Cammack et al., 2015). This community flourishes shortly after the physiological and immunological functions of the host have ceased: the final consequence is a recycling process that transforms the remaining nutrient and energy pulses into new life (Benbow et al., 2019). Thereby, all three life domains are involved: Archaea, Bacteria and Eukarya (i.e., all animals, plants and fungi) (Figure 1) (Benbow et al., 2015b; Polis et al., 1997; Swift et al., 1979).Summary of phylogenetic relationships for biomass, which includes all three life domains: Archaea (e.g., Sulfolobus, a species known from geysers in Yellowstone Park), Bacteria (e.g., Escherichia coli, a species well known from the lower intestine of warm-blooded organisms) and Eukarya. Within the domain Eukarya we find besides Homo sapiens all other animals, plants such as Hoodia gordonii (a flower that smells like rotten flesh and thus mainly attracts necrophagous flies) and fungi such as Cryptococcus neoformans. The kingdom Animalia includes, among other Phyla, arthropods, which potentially comprise more than 5 million species on earth. The presumed numbers of species and Phyla were taken from an online database (GBIF.org, 2017). Figure 4 will continue the phylogenetic tree focusing on Diptera.

Carrion ecosystem

Vertebrates consist of soft and hard tissues, which are called carrion after the death of the animal. Hard tissues such as bones, hairs and teeth resist the decomposition processes and can therefore be found even after a long period has passed since the animal’s death (Janaway et al., 2009). The ecosystem carrion becomes anoxic and the same enzymes that contributed to the metabolic function in the living animal start to decompose the soft tissues. Additional enzymes can be transferred by insects and microorganisms (primarily bacteria) (Carter et al., 2010). The majority of microorganisms migrate from the host’s intestinal tract to the lymphatic system and transform macromolecules, such as proteins and lipids, into simpler compounds such as acids and gases. These gases (e.g., volatile organic compounds) are responsible for the typical odours that are associated with putrefaction and bloating of carcasses (Forbes and Carter, 2015). They attract carrion insects, who have evolved strategies to detect and quickly locate necromass (Janzen, 1977; Wall and Warnes, 1994; Dekeirsschieter et al., 2009; Johansen et al., 2014).
Decomposing carrion can be described as ephemeral resource that presents a valuable food source for many different invertebrates and microorganisms that gather on particular spots of the necromass (Doube, 1987; Forbes and Carter, 2015). These spots are mostly characterised by high humidity, little hair and low illuminance (Figure 2) (Archer and Elgar, 2003; Charabidze et al., 2015). Moreover, these invertebrates work against time, since its food-source dissolves bit by bit. Within 7 days, insects and microbes can consume 85% of a 50 kg carcass (Spicka et al., 2011). The remaining soft tissue dries out quickly and becomes a leathery texture, which is difficult or even impossible for most larvae and scavengers to digest. The fact that flies and their larvae avoid dry tissue is demonstrated by the rapid decrease in oviposition shortly after the host’s death, when it begins to dry out (Archer and Elgar, 2003). On the contrary, some necrophagous mites (Acari) and beetles (especially from the family Dermestidae, Silphidae, Nitidulidae, Ptinidae and Cleridae) are interested in such dry remains (Merritt and De Jong, 2015).

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Life cycle of flies and physiology of larvae

Once female blowflies have located a carcass, they lay egg batches in sheltered places close to fleshes that are easy to digest (e.g., nostrils and eyes; Byrd and Castner, 2001) or where already several other egg batches were laid (Serra et al. 2010) (Figure 7). Each batch usually contains between 100 and 300 eggs from which small larvae hatch (Smith 1986). This marks the beginning of a new life cycle (Figure 8). Blowflies (Diptera: Calliphoridae) represent in general the first insects that colonise a carcass A Two Lucilia sp. flies with their ovipositor extended to lay eggs on the foot of a pig carcass, where other egg batches have already been laid. In addition, there are already 3rd-stage larvae under the foot and hind leg. B Two clearly different fly species (one of the genus Lucilia and one Protophormia) have settled for oviposition on the same spot of a rat carcass. The life cycle of a blowfly is defined by four morphologically distinct life-stages: egg, larva, pupa and imago. Eggs and pupae constitute immobile development stages, whereas larvae and adults constitute active (i.e., moving) stages. However, while flies can fly up to 9 km/h and cover a flying distance of several km, larvae crawl only up to 0.017 km/h (i.e., 28 cm/min) and up to 30 m away from their food source if necessary (Bomphrey et al., 2009; Braack, 1981; Charabidze et al., 2008; Green, 1951). Thus, flies select the carcass for their eggs and the larvae their aggregation site at the carcass.
Each necrophagous larva completes three larval instars, between which the larva sheds its skin (a process called ecdysis, representing a transition to a new physiological stage), until finally the last outer skin shrinks and hardens to a puparium (Castner, 2001; Gunn, 2009). To enter the next development stage, threshold size and weight must be met: consequently, a sufficient food intake during the feeding larval stages is mandatory (Hightower et al., 1972; Shaaya and Levenbook, 1982). Once peakfeeding is reached, necrophagous maggots usually.

Table of contents :

Résumé
Abstract
Zusammenfassung
Acknowledgments
List of figures
List of tables
1 Introduction 
1.1. Carrion ecology
Necromass and Necrobiome
Carrion ecosystem
Carrion feeders & breeders
1.2. Blowflies
Phylogenetic relationships
Life cycle of flies and physiology of larvae
1.3. Abiotic and biotic interactions
Differences in food
Differences in temperature
1.4. Selective pressures on carrions and larval development strategies
1.5. Goals and Hypothesis
2 Behavioural Study 
2.1 Material and methods
Insect rearing
Experimental setup
Statistical analysis
2.2 Results
2.3 Discussion
3 Development Study 
3.1 Material and methods
Preparation and start of experiments
Test series and procedure
Development rate and statistical analysis
3.2 Results
Mutual benefits at an intermediate temperature
Positive and negative density dependence
Balance between feeding and postfeeding time
Costs of fast development
3.3 Discussion
Self–critique
Benefits of aggregation
Mechanisms of mutualism
Balance between feeding and postfeeding time
Costs of fast development
4 Conclusion 
4.1. Adaptive ecology and novelties of this thesis
4.2. Publications
5 Supplementary material 
5.1. Additional behavioural experiments
5.2. Deeper look at the species–specific mortality
5.3. Forensic entomology perspective
Principle of minimum postmortem interval estimation
A meta–analysis of migration and eclosion time
Critique
References

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