**7. Conclusions**

*Life Cycle and Development of Diptera*

**study**

[97] 2009 SC MG 61/na

[98] 2010 SC MG 30/na [63] 2005–2010 FL ST, MT 43/na

[46] 2010 FL SG 14/na [57] 2014 SC GF 1/na [100] 2012, 2013 SC MG, GM 127/na

[101] 2013, 2014 SC VGF 11/na [64] 2015 SC GF 2/na [102] 2013 SC MG, GM 37/na

VGF

FL ST, MT, SG 254/na

SC GM 131/na

FL ST, MT 27/na

*The islands are abbreviated as Santa Cruz (SC), Floreana (FL), Isabela (IS), Daphne Major (DMj). The 'total number of nests examined' refers to all active nests monitored over the course of the study and 'number inspected during egg phase' is the sample size for the sub-set of nests examined during host incubation (usually following abandonment or predation) where 'na' denotes that nests have been not sampled during the egg phase. The column 'P. downsi larvae during the egg phase' states 'yes/no' referring only to nest inspections that occurred during the egg phase. Host species are abbreviated as small tree finch (ST), large tree finch (Camarhynchus psittacula) (LT), small ground finch (SG), medium ground finch (MG), woodpecker finch (Cactospiza pallida) (WP), warbler finch (Certhidea olivacea) (WF), cactus finch (Geospiza scandens) (CF), Galápagos mockingbird (GM), smooth billed ani (Crotophaga ani) (SBA), yellow warbler (Dendroica petechia) (YW), dark billed cuckoo (Coccyzus melacoryphus) (DBC), vermillion flycatcher (Pyrocephalus rubinus) (VF), vegetarian finch (Platyspiza crassirostris) (VGF), and Galápagos flycatcher (Myiarchus magnirostris) (GF).*

*Evidence of Philornis downsi larvae present in nests during incubation and before nestling hatching in studies* 

**Island Host species Total no. of nests** 

[88] 2010 SC MG 43/na Female MG in parasitised

**examined/no. inspected during egg phase**

FL ST, MT, SG 561/na Evidence that *P. downsi*

SC ST, WF 850/177 Yes Larvae and puparia

26/na

*P. downsi* **larvae during the egg phase**

**Comments**

nests had more *P. downsi* antibodies and spent more time standing upright when brooding than non-parasitised nests

oviposition behaviour occurred more synchronously and earlier in nesting phase in later years of the study

found in 18/72 ST nests and 52/105 WF nests that failed during egg phase; range in prevalence across species and years was 0–80% of nests

**Ref # Year (s) of** 

[54] 2004, 2006,

[58] 2004, 2005,

[86] 2012, 2014,

[103] 2012, 2013,

[104] 2010, 2013,

2008, 2010, 2012, 2013

2006, 2008, 2010, 2012, 2013, 2014

2015, 2016, 2017

2015, 2016

2014

[99] 2013 SC ST, SG, MG,

**62**

**Table 2.**

*on the Galápagos Islands.*

As one of three avian nest parasitic genera in Diptera, the genus *Philornis* provides a useful system to explore shifts in larval feeding behaviour in native and invasive species. *Philornis downsi* has been accidentally introduced to the Galápagos Islands and first observed in the nests of Galápagos land birds in 1997. In this chapter, we explored similarities and differences between *P. downsi* larval development and behaviour with what is known from the other 52 *Philornis* species. More basal *Philornis* (*aitkeni*-group) species have free-living coprophagous larvae and more recently evolved *Philornis* (*angustifrons*-group) tend to have subcutaneous hematophagous larvae with the exception of *P. downsi* that has free-living semi-hematophagous larvae. Since its introduction to the Galápagos Islands, there have been documented changes in the behaviour of *P. downsi*. During the early years after initial discovery of *P. downsi* on the Galápagos Islands, oviposition behaviour was asynchronous across the nesting cycle and larvae appeared to have fed exclusively on developing nestlings until 2005. In later years, *P. downsi* oviposition behaviour was earlier in the nesting cycle and more synchronous, and since 2006, larvae have also been recorded to feed on incubating females. The first records of *P. downsi* larvae in host nests with eggs rather than hatchlings occurred at the end of a four-year drought on the Galápagos in 2006. Since 2012, up to 80% of host nests with eggs may contain *P. downsi* larvae on Santa Cruz Island. Larval feeding by *P. downsi* on adult birds has been observed in laboratory finches and in one *Philornis* system (species unknown) in Puerto Rico. In light of changes in *P. downsi* larval feeding behaviour, we provided a description and photos of the larval instars for use in field identification. We compiled the observations to date of *Philornis* behaviour and ontogeny within a broad taxonomic framework and summarised patterns of change in the oviposition behaviour of *P. downsi* in its (presumably) novel habitat on the Galápagos Islands. By examining *P. downsi* in relation to other *Philornis* species, we provided a broad phylogenetic context for the potential behavioural repertoire of an invasive species under conditions of intense natural selection in a novel environment.

#### **Acknowledgements**

We thank the Galápagos National Park authority for research permits and the opportunity to work on the Galápagos, and the Charles Darwin Research Station for logistical support. We thank Charlotte Causton, Paola Lahuatte, Birgit Fessl, George Heimpel and Arno Cimadom for their useful comments on the manuscript. We thank Bradley Sinclair for advice on larval instar morphology. We thank Justin Holder, Grant Gully and Ben Parslow for their assistance with the photographs and guidance on using the Visionary System. This publication is contribution number 2277 of the Charles Darwin Foundation for the Galápagos Islands.
