Small Tortoiseshell

Aglais urticae (ag-LAR-iss ur-TY-see)

Small Tortoiseshell, Wiggonholt (31 August 2011)
Photo © Mark Colvin
 

Wingspan
Male: 45 - 55mm
Female: 52 - 62mm

Checklist Number
59.027

Family:NymphalidaeRafinesque, 1815
Subfamily:NymphalinaeRafinesque, 1815
Tribe:NymphaliniRafinesque, 1815
Genus:AglaisDalman, 1816
Subgenus:  
Species:urticae(Linnaeus, 1758)

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Introduction

The Small Tortoiseshell is one of our most-familiar butterflies, appearing in gardens throughout the British Isles. Unfortunately, this butterfly has suffered a worrying decline, especially in the south, over the last few years. This butterfly has always fluctuated in numbers, but the cause of a recent decline is not yet known, although various theories have been proposed. One is the increasing presence of a particular parasitic fly, Sturmia bella, due to global warming - this species being common on the continent. The fly lays its eggs on leaves of the foodplant, close to where larvae are feeding. The tiny eggs are then eaten whole by the larvae and the grubs that emerge feed on the insides of their host, avoiding the vital organs. A fly grub eventually kills its host and emerges from either the fully-grown larva or pupa before itself pupating. Although the fly attacks related species, such as the Peacock and Red Admiral, it is believed that the lifecycle of the Small Tortoiseshell is better-synchronised with that of the fly and it is therefore more prone to parasitism. This is one of our most widespread butterflies, occurring throughout the British Isles, including Orkney and Shetland.

Aglais urticae

This species was first defined in Linnaeus (1758) as shown here (type locality: Sweden).

Small Tortoiseshell -  Cornwall Coast Path 12-Sept-2013

Male
Photo © Neil Hulme

Small Tortoiseshell - Chaldon, Surrey 14-June-09

Male Underside
Photo © Vince Massimo

Small Tortoiseshell - Ferring Rife, Sussex 22-April-2013

Female
Photo © Neil Hulme

Small Tortoiseshell ovipositing - Coulsdon, Surrey 14-July-2013

Female Underside
Photo © Vince Massimo

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Conservation Status

Despite being a widespread and common species in certain areas, the declining fortunes of this butterfly, especially in the south, mean that this butterfly is a species of conservation concern.

UK BAP StatusDistribution Trend (%)Population Trend (%)
Not Listed
Stable-3
Large Decrease-64

The table above shows the distribution and population trends of species regularly found in the British Isles. The distribution trend represents a comparison between data for the periods 1995-1999 and 2005-2009. The information provided is taken from the Butterfly Conservation report The State of the UK's Butterflies 2011. The UK BAP status is taken from the UK Biodiversity Action Plan (UK BAP) (2007 review).

Habitat

This butterfly can turn up almost anywhere, from city centres to mountain tops. As such, it is one of our most successful butterflies. It is most-often seen, however, where nettles grow in abundance, such as field margins. This butterfly is often encountered while hibernating in an outbuilding, such as a garage, shed or barn, where they may be found in the company of other individuals. Other hibernation sites include hollow trees and wood piles.

Distribution

 

Click here to see the distribution of this species or here to see the distribution of this species together with specific site information overlaid.

Life Cycle

The adult butterflies can be seen at any time of the year, even on the last days of December or first days of January if the temperature is high enough to wake them from hibernation. However, adults normally emerge from hibernation at the end of March and start of April. There are typically 2 broods each year, except in the north, where there is usually only a single brood. Whether single or double-brooded, the butterfly is a familiar sight in late summer as it takes nectar to build up essential fats in preparation for hibernation.

The chart(s) above have been correlated with the phenology plot below, taken from the UK Butterfly Monitoring Scheme. The blue line gives average counts over the full data set from 1976 to date, and the red line gives the average for the last year.

Imago

The two sexes are almost identical in appearance, with the distinctive yellow and orange uppersides providing a contrast with the drab undersides that provide the butterfly a good deal of camouflage when hibernating.

In the afternoon, males set up territories, usually close to a nettle patch, where they rest of the foodplant or ground with their wings open, waiting for a passing female. When a female enters the territory, a most curious courtship begins. The male approaches the female from behind and starts to "drum" his antennae on the hindwings of the female, making a feint sound that is audible to the human ear. The female may fly a little distance, with the male following, where the process repeats. This can go on for several hours with the couple spending a good amount of time basking together. Eventually, usually in early evening, the female will lead the male into vegetation, often a nettle patch, and crawl between stems with the male following, where they eventually mate. They remain coupled until the following morning.

Adults feed primarily on Betony (Stachys officinalis), Bramble (Rubus fruticosus), Carline Thistle (Carlina vulgaris), Dandelion (Taraxacum agg.), Devil's-bit Scabious (Succisa pratensis), Field Scabious (Knautia arvensis), Greater Stitchwort (Stellaria holostea), Hawkweeds (Hieracium/Hypochoeris), Heather (Calluna vulgaris / Erica spp.), Hemp Agrimony (Eupatorium cannabinum), Ivy (Hedera helix), Knapweeds (Centaurea spp.), Marjoram (Origanum vulgare), Primrose (Primula vulgaris), Privet (Ligustrum vulgare), Ragwort (Senecio jacobaea), Thistles (Cirsium spp. and Carduus spp.), Thyme (Thymus polytrichus) and Water Mint (Mentha aquatica).

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Ovum

The female is quite choosy about where she lays, which is not surprising since she lays her green eggs in large batches and needs to ensure that the location is just right for the developing larvae. Typical sites are nettle patches containing relatively-new growth, and that receive full sun. Eggs are laid on the underside of a nettle leaf, usually one near the top of the stem and at the edge of the nettle patch. The eggs are not laid in neat rows, but piled on top of one another. Egg batches can contain up to 80 eggs which take some time to lay - typically between 20 and 90 minutes. The egg stage lasts between 1 and 3 weeks, depending on the weather.

Small Tortoiseshell - ovum - Cumnor, Oxford - 25-Apr-14-14

Photo © Pete Eeles
25-Apr-2014

Small Tortoiseshell - Ova - 18-April-2015 - Bishopstone, East Sussex

Photo © Butterflysaurus rex

Small Tortoiseshell Ova - Somerset - 29/06/13

Photo © William
29-Jun-2013

Small Tortoiseshell ova. 29/3/2014. Seaford. E.Sussex.

Photo © badgerbob
29-Mar-2014

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Larva

On emerging from their eggs, the larvae build a communal web, usually at the top of the nettle, from which they emerge to bask and feed. As the larvae grow, they move to new plants, building new webs along the way. This leaves a trail of webs, decorated with shed larval skins and droppings, that show the passing of time, and allows the patient observer to trace the larvae all the way back to the plant where the eggs were laid. The first experience that some people have of a Small Tortoiseshell is seeing these webs as they extend over stretches of nettle, with the larvae resting communally and quite visibly on the surface of the web, or feeding from nearby leaves.

Larvae have several techniques to avoid predation. When disturbed, a group of larvae will often jerk their bodies from side to side in unison, which must be a formidable sight to any predator. The larvae will also regurgitate green fluid and will, if necessary, curl up in a ball and drop to the ground. Larvae feed by both day and night and there are 4 moults in total.

The primary larval foodplants are Common Nettle (Urtica dioica) and Small Nettle (Urtica urens).

Small Tortoiseshell Larvae - Bishopstone - East Sussex - 29th-April-2014

Photo © Butterflysaurus rex

Parasitic Fly (Phryxe vulgaris or Phryxe nemea) - Caterham, Surrey 8-July-2013

Photo © Vince Massimo
08-Jul-2013

Pupa of parasitic fly (likely Sturmia bella) - Caterham, Surrey 18-June-2013

Photo © Vince Massimo
18-Jun-2013

Small Tortoiseshell - larva - Ashford Hill NNR - Uknown date [Tim Norriss]

Photo © Tim Norriss

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Pupa

The larvae disperse as they become fully grown, and eventually wander off to find a suitable pupation site. The pupa is formed head down, attached to a stem or leaf by the cremaster. The colour of the pupa is quite variable, often having a beautiful metallic sheen. This stage lasts between 2 and 4 weeks, depending on temperature.

Small Tortoiseshell pupa - Caterham, Surrey 22-June-2013

Photo © Vince Massimo
20-Jun-2013

Small Tortoiseshell pupa - Caterham, Surrey 22-June-2013

Photo © Vince Massimo
22-Jun-2013

Small Tortoiseshell - pupa - Unknown location - 30-May-05 [REARED] [Matt Berry]

Photo © Matt Berry

Small Tortoiseshell Pupa - Somerset - 03/08/13

Photo © William
03-Aug-2013

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Aberrations

This species displays considerable variation in both the upperside ground colour and the familiar upperside pattern and markings.

Much work has been carried out in this species to discover how temperature shock in the late larval/ early pupal stage can affect the development of pigments in the imago. These experiments have assisted our understanding of how and when the pigments are 'mapped' in the Vanessid butterflies. The Small Tortoiseshell appears to be particularly sensitive to temperature shock, and this species has been widely bred in the past in the hope of producing extreme environmentally controlled aberrations such as ab. semi-ichnusoides.

Extreme shocks of either heat or cold during the last 24 hours of the larval stage and the first 48 hours of the pupal stage can disrupt the natural process of metamorphosis and inhibit the normal processes in which organic chemicals create the colouration of the wing scales.

The expression of aberration through temperature shock is a graduated process; one level of exposure creates ab. semi-ichnusoides, a little more fuses all three black blotches along the costa of the forewing to create ab. conjuncta, and the most extreme exposure produces an almost entirely melanic form known as ab. osborni. The conditions required to produce the latter forms are thought to be so extreme that they lie right on the very cusp of extremes of temperature that would be fatal to the larva or pupa. Consequently these forms are rare in captivity and extremely unlikely to be met with in the wild.

It is difficult to ascertain how frequently any of these aberration occur in the wild, however it is a rare event that exposes the newly formed pupa or transitional larva to the necessary conditions for metamorphosis to be disrupted in this way, and this is supported by the paucity of historical sightings of the extreme aberrations in the wild. That is not to say that the lesser of the aberrations do not occur however, and an ab. semi-ichnusoides presents a particularly striking sight when nectaring on a garden buddleia amongst typical individuals.

It has been speculated that severe late frosts could possibly cause instances of these aberrant forms, as well as a larva/pupa being exposed to particularly strong sunlight after having the normally sheltered pupation site disturbed in some manner.

Environmental influences are not the sole cause of aberration in this species however, and there are many genetic aberrations which the observer could encounter at any time. Occasionally the orange ground colour is replaced entirely by a pale whitish buff (ab. lutea), as well as various transitional shades leading to this. Similarly, sometimes the orange colour is replaced by a brownish-purple hue and this is known as ab. brunneoviolacea.

There are in excess of 105 named aberrations known to occur in Britain.

Click here to see the aberration descriptions and images for this species.

Similar Species

Large Tortoiseshell

Description to be completed.

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The following links provide additional information on this butterfly.

References

The species description provided here references the following publications:

ReferenceDetails
Dalman (1816) Dalman, J.W. (1816) Kongl. Svenska Vetenskaps akademiens Handlingar.
Linnaeus (1758) Linnaeus, C. (1758) Systema Naturae. Edition 10.
Rafinesque (1815) Rafinesque, C.S. (1815) Analyse de la nature ou Tableau de l'univers et des corps organisés.