Hover Fly 

Ischiodon scutellaris (Fabricius, 1805)       

A brief overview of Ischiodon scutellaris 

Ischiodon scutellaris is a species of hover fly (Diptera: Syrphidae) that is known to provide ecosystem services by pollinating flowers during the adult stage¹ and acts as a biological control agent by feeding on aphids during the larval stage² . It resembles bees or wasps morphologically³ , a phenomenon known as Batesian mimicry that is common among hover flies and aids in deterring predation⁴ (Figure 1).

There are also many interesting aspects to the taxonomy and systematics of I. scutellaris. The genus Ischiodon has a widespread distribution despite only having four species (I. scutellaris, I. aegyptius, I. astales, I. feae). Each species has a distinct distribution and only I. scutellaris is found in Singapore. The genus Ischiodon has a confusing taxonomic history where its validity as a genus was questioned and I. scutellaris has many synonyms because its genus Ischiodon is morphologically similar to many syrphid genera⁵ . The phylogenetic position of I. scutellaris in the Family Syrphidae was only determined confidently in 2018⁶ .

What is a hover fly?

The common term hover fly is used to refer to a group of flies that are commonly seen hovering around flowers during pollination. Other common names include flower flies or syrphid flies, with the latter referring to their scientific classification in the Family Syrphidae¹ .

A hover fly "hovering" around flowers.  Video by Ronnie Photography. Taken from YouTube under Fair Use guidelines.

The Family Syrphidae is one of the most diverse families in the Order Diptera, having almost 6000 described species and occurring in almost all parts of the world⁷ . Most hover flies are brightly-coloured and thus often mistaken for bees or wasps due to morphological similarities³ . This phenomenon is known as Batesian mimicry - a term used to describe the situation where a palatable species looks similar to a non-palatable species in order to escape predation⁴ . By looking similar to bees or wasps that sting, hover flies may deter potential predators. Hover flies have one pair of wings, with “the other pair” modified into balancing organs known as halteres⁸ . They can be easily distinguished from other Diptera from the presence of a false vein in their wings³ . Hover flies are widely recognized for providing important ecosystem services from their role as pollinators and biological control agents in the environment⁹ .

Life cycle and associated biology in each stage

Hover flies are holometabolous insects, meaning their life cycle consists of four stages: egg, larvae, pupa and adult⁸ .

Note: the description in this section is specific to I. scutellaris but the pictures may not necessarily point to the species. In addition, the duration of each stage in the life cycle of I. scutellaris was based on laboratory experiments¹⁰ . Future work may be conducted to assess their developmental duration in the wild. 

Egg stage

The egg stage of I. scutellaris lasts for three to four days¹⁰ . The egg of I. scutellaris is chalky white in colour but becomes light purple once hatching is imminent. The egg has an oblong-oval shape with the ends bluntly rounded. The size of an egg is approximately 0.76 - 0.81mm in length and 0.27 - 0.31mm in width¹¹ (Figure 2).

Figure 2. Egg of a hover fly next to an aphid (may not be that of I. scutellaris).

Source: Brenda Dobbs on Flickr, licensed under CC BY-NC 2.0. Annotations by Tan Jia Wei.

Larval stage

The larval stage of I. scutellaris is approximately five to seven days, with the larvae having three instars¹⁰ . The larvae of I. scutellaris has an elongated shape with wrinkled sides. It has a greenish-brown colour, with a median white line that spans across the entire body length. Brown spots flank the median white line and are also present on the lateral sides of the body. The larvae is roughly 9.6 - 10.8mm in length and 2.2 - 2.3mm in width¹¹ (Figure 3).

Figure 3. Larvae belonging to the Tribe Syrphini (that I. scutellaris belongs in)

Source: Sara “Asher” Morris on Flickr, licensed under CC BY-NC 2.0.

Ischiodon scutellaris larvae as biological control agent

Hover flies belonging to the subfamily Syrphinae, which includes I. scutellaris, are known for larvae that are specialised predators of aphids¹² . Aphids are considered pests that cause destructive damages to crop plants and of great economic concern³ (Figure 4).

Figure 4. A hover fly larvae (may not be I. scutellaris) that ingested aphids.

Source: Mark Yokoyama on Flickr, licensed under CC BY-NC-ND 2.0.

Ischiodon scutellaris has been recorded in India to be feeding on various aphid species in the genus Aphis (Aphis craccivora, A. gossypii, A. spiraecola and A. verbasci), Acyrthosiphon pisum, Brevicoryne brassicae, Lipaphis erysimi and Myzus persicae etc² . A later study included the Pearl Millet aphid, Rhopalosiphum maidis, into the list of aphid species that I. scutellaris larvae feed on¹³ . Ischiodon scutellaris also has been recorded to be found on host plants in India such as cabbage, chrysanthemum, cotton, crucifers, mustard, radish, redgram, rose, watermelon and wheat etc³ . The species was feeding on aphids on these plants³ .

With a voracious appetite for aphids, I. scutellaris and other species in the subfamily Syrphinae have the potential to become effective biological control agent of aphid crop pests¹² . Ischiodon scutellaris was hailed as one of the six hover flies with top economic importance in terms of being biological control of aphids in a field study conducted in India¹¹ .

There is currently no literature on host plants and prey items of I. scutellaris in Singapore. However, records of such information from India may aid in understanding or predicting the species’ biology in Singapore as both countries experience tropical climate conditions.

Pupal stage

The pupal stage of I. scutellaris ranges from four to twelve days¹⁰ . The pupa is initially brown in colour, but becomes transparent with visible abdominal bands once it is ready to emerge as an adult. The pupa is about 5.8 - 6.8mm in length and 2.0 - 2.5mm in width¹¹ (Figure 5).

Figure 5. Pupa belonging to the subfamily Syrphinae (that I. scutellaris belongs in)

Source: gbohne on Flickr, licensed under CC BY-SA 2.0.

Parasitism relationship

Ischiodon scutellaris is known to be parasitized by an Ichneumonid wasp, Diplazon laetatorius (Hymenoptera: Ichneumonidae)¹⁰ . This species of ichneumonid wasp is an endoparasite, where the female will oviposit in the egg or larvae (1^st or 2^nd instar) of the host organism and the adult parasitoid wasp will emerge from the pupa of the host organism¹² (Figure 6).

Figure 6. Diplazon laetatorius on a leaf.

Source: Nikk on Flickr, licensed under CC BY 2.0.

Adult stage

The lifespan of adult I. scutellaris differs between the sexes, ranging from seven to eight days for adult male and eleven to twenty-two days for female¹⁰ . The description for adult I. scutellaris is provided under the taxonomy section.

Adult hover fly as pollinator

Along with bees and butterflies, hover flies are also avid pollinators of flowers¹⁴ . They rely on visual and olfactory cues to locate suitable flowers¹⁵ . Adult hover flies in the subfamily Syrphinae, which includes I. scutellaris, have been recorded to feed on nectar and pollen in flowers, and honeydew synthesised by aphids¹² . Pollen in particular is crucial for ovary development in female hover flies¹ (Figure 7).

Figure 7. Ischiodon scutellaris on a flower.

Source: Sam Dickinson on Flickr, licensed under CC BY-NC-ND 2.0.

Interestingly, a study has suggested that bees are able to infer if a flower has been visited by hover flies based on leftover chemical cues. This may help bees to avoid resource-depleted flowers and in turn, increase overall efficiency in pollinating flowers¹⁶ .

Hover flies act as pollinators in a wide range of ecological communities and range from general pollinators to specialized pollinators. This implies that hover flies may contribute to the reproductive success of plants¹ , and thus maintain ecosystem services and crop production¹⁴ . With the global decline in honey bees and other bee species in recent years¹⁷ and the fact that hover flies are known to be active all year around⁸ , pollination may become more dependent on hover flies¹⁷ in the future.

Batesian mimicry

Hover flies are often mistaken as Hymenoptera (bees, wasps and others) at first glance due to morphological similarities, in terms of body size, colour and shape. It is hypothesised that hover flies mimic Hymenoptera to avoid predatory attacks by birds. This phenomenon is also known as “Batesian mimicry” – a term used to describe the situation where a palatable species looks similar to a non-palatable species in order to escape predation. Batesian mimicry is widespread in insects, and especially prevalent in hover flies. In addition to morphological similarities, there are also records of hover flies mimicking the behaviour of Hymenoptera, such as visiting flowers at similar timings⁴ .

Despite morphological similarity, hover flies and bees/wasps belong to different insect taxa. Hover flies can be easily distinguished from bees and wasps based on the antennae or number of paired wings. Bees or wasps have longer antennae than hover flies. In addition, bees or wasps have two pairs of wings while hover flies have one pair of wings, with “the other pair” modified into balancing organs known as halteres⁸ (Figure 8).

Figure 8. Comparison between a Hymenoptera (wasp) and a Diptera (hover fly). 

Left. A three-banded Mason wasp, Ancistrocerus trifasciatus. Source: Donald Hobern on Flickr, licensed under CC BY 2.0. Annotations by Tan Jia Wei

Right. A hover fly, Ischiodon scutellaris. Source: satish nikam on Flickr, licensed under CC BY NC-SA 2.0. Annotations by Tan Jia Wei.

Taxonomy 

Original description

Ischiodon scutellaris was first described in Systema antliatorum in 1805 by Johan Christian Fabricius as Scaeva scutellaris. There was no mention of the species concept used to confer the species status (Figure 9).

Figure 9. Screenshot of original species description. Annotation by Tan Jia Wei.

Source: Biodiversity Heritage Library under Fair Use.

Etymology

The genus name Ischiodon consists of the neuter adjective ischion derived from the Greek word ischion which means “hips, coxa”, and the masculine Greek noun odontos or odous (odon) which means “tooth”. Combined together, Ischiodon refers to the spur on the metatrochanter in some species⁵ .

Morphological description   

The genus Ischiodon currently consists of a total of four species (I. scutellaris, I. aegyptius, I. astales, I. feae) that ranges from small to medium sizes, and have a slender outlook. The face is yellow-coloured and eyes are bare. For females, the eyes are separated in the middle while for males, the eyes meet together. The thorax is black with yellow streaks at the lateral sides. The scutellum is yellow and slightly brownish on the disc. The abdomen is elongated, with the areas in black becoming yellowish or reddish towards the abdominal end⁵ .

Ischiodon scutellaris may be distinguished from I. astales and I. feae by having a short calcar (refer to Figure 10) on the metatrochanter, and from I. aegyptius by having symmetrical claws in all legs⁵ (Figure 10).

Figure 10. Lateral, dorsal and frontal view of I. scutellaris.

Specimen collected by Toh Kai Xin and property of Insect Diversity Lab. Image by Tiffany Lum (all permission obtained).

Annotations by Tan Jia Wei.

Distribution

Ischiodon scutellaris is found in Caucasus, Greece, Iran, Kazakhstan, Arabian Peninsula southern to Indomalayan Region, Japan, Australasian Region and Oriental Regions⁵ , which includes Singapore.

The genus Ischiodon is widespread and occurs in Southeast Asia, Australia, Southern Europe, Africa, China, Japan, India, the Mediterranean basin and certain Pacific Islands. All four species are known to have distinct distribution with little overlaps. Ischiodon aegyptius is found in Southern Europe and Africa, while I. feae is an endemic species on the Cape Verde Islands. Ischiodon astales was discovered recently in Madagascar⁵ .

Type specimen

The syntype of Scaeva scutellaris (now Ischiodon scutellaris) is kept in the Zoological Museum at the University of Copenhagen, Copenhagen, Denmark⁵ . A syntype refers to a specimen in a series of specimens used to describe the species. An image of the syntype is not available.

Species concept

In the original description by Fabricius (1805), there was no mention of the species concept used to delimit the species. This page is of the opinion that the Phylogenetic species concept (sensu Wheeler & Platnick (2000))¹⁸ may be used to delimit I. scutellaris as a species. The species concept confers species status to a species once a population has unique character states that differ from another population. This species concept is also able to address allopatric populations, which is relevant for the genus Ischiodon where all four species have distinct distributions with minimal overlaps.

Characters such as a short calcar on the metatrochanter, symmetrical claws in all legs and male genitalia shape may be used to delimit I. scutellaris as a species. Ischiodon scutellaris may be distinguished from I. astales and I. feae via morphological differences such as the presence of a short calcar on the metatrochanter and from I. aegyptius via morphological difference such as having symmetrical claws in all legs⁵ .

Synonyms

Ischiodon scutellaris has many synonyms because the genus Ischiodon is morphologically similar to many syrphid genera⁵ .

Below is a list of synonyms given to Ischiodon scutellaris⁵ .

Scaeva scutellaris (Fabricius, 1805)

Syrphus coromandelensis (Macquart, 1842)

Sphaerophoria annulipes (Macquart, 1855)

Syrphus splendens (Doleschall, 1856)

Syrphus nodalis (Thomson, 1869)

Syrphus erythropygus (Bigot, 1884)

Syrphus ruficauda (Bigot, 1884)

Melithreptus novaeguineae (Kertesz, 1899)

Ischiodon trochanterica (Sack, 1913)

Melithreptus ogasawarensis (Matsumura, 1916)

Ischiodon boninensis (Matsumura, 1919)

Epistrophe platychiroides (Frey, 1946)

Ischiodon penicillatus (Hardy, 1952)

Epistrophe magnicornis (Shiraki, 1963)

Sphaerophoria macquarti (Goot, 1964)

Phylogeny

Phylogenetic study in 2018

A phylogenetic study in 2018⁶ used molecular data, which included the mitochondrial gene cytochrome c oxidase sub-unit I (COI), nuclear 18S and 28S ribosomal RNA genes to ascertain phylogenetic relationships of hover flies in the Scaeva and Eupeodes genera, and certain genera or subgenera in the tribe Syrphini, including the Ischiodon and Simosyrphus genera. DNA was extracted from all 108 samples and sequenced using Polymerase chain reaction (PCR).

The COI gene sequence was aligned manually and gaps were not inserted into the sequence. Secondary structures of nuclear 18S and 28S ribosomal RNA genes were used to insert gaps into the sequence datasets. The alignment procedure is made available online, with links provided in the study. The molecular data from different genes were combined and analysed for phylogenetic relationship using the optimality criterias maximum likelihood and Bayesian inference. For both analysis, the data in the combined dataset was partitioned into five sets – first codon position for COI gene, second codon position for COI gene, third codon position for COI gene, nuclear 28S gene and nuclear 18S gene. Different models were used for each partition. The step is well done because mitochondrial genes evolve faster than ribosomal genes and third codon position evolves faster than first or second codon position for mitochondrial genes, thus different molecular models should be used in order to reach a more confident conclusion for phylogenetic analysis.

The maximum likelihood tree with the highest likelihood score is presented in Figure 11. Node support values were attained using bootstrapping method that ran 1000 times. As two samples of I. scutellaris were included in the analysis, one gave a bootstrap value of 100 while the other provided a bootstrap value between 95 to 99. As such, the phylogenetic position of I. scutellaris as sister group to Simosyrphus and Scaeva genera is confident based on the maximum likelihood optimality criteria. The results from using Bayesian Inference optimality criteria is not discussed on this page (Figure 11).

Figure 11. Phylogenetic tree generated from maximum likelihood and Bayesian inference.

Taken from Mengual et al. (2018) under Fair Use. Annotation in red by Tan Jia Wei.

Phylogenetic study in 2008

Previously in 2008, a study¹⁹ made use of molecular data, which include COI mitochondrial gene and 28S ribosomal RNA gene, to ascertain the relationship within tribes in the subfamily Syrphinae. DNA from all 98 samples were extracted and sequenced using PCR. The combined molecular dataset was subjected to direct optimisation using the program POY, which also computed Bremer support values for nodes on the most parsimonious tree. This page is of the opinion that direct optimisation is a debatable method because it is computationally difficult to conduct and the workflow is intractable. However, it is a one-step process as it combines alignment step and tree construction step, which were previously done one after the other. Bremer support values range from 1 to 5, with 3 implying good support and 5 indicating high support²⁰ . The support values indicate how many additional steps are required to forgo a branch in the consensus tree from multiple most parsimonious trees²¹ .

Note: Ischiodon scutellaris was referred to as Simosyrphus scutellaris in this phylogenetic tree published in 2008 because there was a proposal in 2006 to synonymise the Simosyrphus and Ischiodon genera based on similar larval and pupa morphologies and molecular evidence²² . This highlights how species nomenclature has an information filing and retrieval function and how taxonomic actions may have subsequent effects on phylogenetic works. The validity of the genus Ischiodon has been debated due to morphological similarity with the syrphid genus Simosyrphus. The dispute first originated when Syrphus grandicornis (now Simosyrphus grandicornis) was synonymized under Ischiodon scutellaris in 1912 despite differences in the male genitalia⁵ . Throughout the years, species in each genus have been placed under each other for a few times. However, the confusion is resolved in 2018 (refer to 2018 study mentioned previously in the phylogeny section) and both Ischiodon and Simosyrphus are valid genera that are sister groups to each other⁶ .

Back to the 2008 phylogenetic study, the tree branch containing I. scutellaris had a Bremer support value of 2, suggesting that the phylogenetic position of the species is not confident. However, its sister group is still consistent with what was found in the 2018 phylogenetic study (Figure 12).

Figure 12. Most parsimonious tree with Bremer support values indicated above branch.

Taken from Mengual, Ståhls & Rojo (2008) under Fair Use. Annotation in red by Tan Jia Wei.

Some concluding statements on the phylogenetic position of Ischiodon scutellaris

Why is the phylogenetic position of I. scutellaris confident in the phylogenetic study in 2018 but not in 2008? Some possible reasons may include number of taxa, number of genes sequenced, the alignment method or the optimality criteria used for the phylogenetic analysis (Table 1). 

Table 1. Comparison between 2018 and 2008 phylogenetic studies which include I. scutellaris. 

In conclusion, to resolve phylogenetic relationships confidently, greater number of genes should be sequenced to attain higher node support values and data should be explored with different techniques.

References