Dragonflies and damselflies

Transcription

MODULE 8: ADVANCED IDENTIFICATION OF
DRAGONFLIES AND DAMSELFLIES
(ODONATA): LARVAE AND EXUVIAE
- BY STEVE BROOKS AND STEVE CHAM
Introduction to this module
There are currently 38 resident
breeding species of
dragonflies in Great Britain
divided between two
suborders: the Anisoptera
which includes 22 species and
Zygoptera, which includes 16
species (plus an additional one
in Ireland). There are a number
of frequent migrants to this
country, of which some are
known to have bred, although
only three are known to have
maintained a breeding colony
for more than a few years. These three species will also be included in this
module. One Anisopteran and two Zygoptera have become extinct in the
last fifty years. Adult dragonflies are relatively easy to identify but the larvae
and exuviae are more problematic, especially the damselflies. These require
critical examination using a 10X magnifier and / or microscope.
As in the other Modules, there are mandatory exercises, which will be
marked by your tutor. You can submit them by printing out this workbook
and filling in the appropriate spaces by hand, or inserting photographs
where relevant (but please add your name to the front of the workbook).
Alternatively, you can use the template appended to the introductory part
and send completed exercises by email. Completion of this module will give
you the Advanced Level for identification of dragonflies.
Module 8: Version 1, January 2009 (Steve Brooks and Steve Cham)
Page 2 of 23
SECTION 1: DRAGONFLIES (ANISOPTERA)
For this section you will need a copy of the British Dragonfly Society
Field guide to the larvae and exuviae of British dragonflies. Volume 1
Dragonflies (Anisoptera). (Cham, S.A. 2007).
In general, only larvae in the final stage can be reliably identified to species
level. Some species have distinctive features that enable identification to
species in all stages, whilst others may only be identified to genus or family.
Larvae in the final stadium can be recognised by the length of the wing
buds, which reach the fourth abdominal segment. You should not attempt
to identify to species level, any larvae in which the wing buds are shorter
than this unless specifically advised. Many of the features, which
characterise each species, are not fully developed in earlier instars. All
dragonfly larvae have ten abdominal segments but the first abdominal
segment is often rather difficult to see, so it is best to count from the last
segment backwards.
The most important features that you will need to examine to identify most
anisopteran larvae are:
• the shape of the body and relative length of the legs
• the shape of the head
• the shape of the mask (labium)
• the number and arrangement of spines on the abdomen
The larvae of Anisoptera are generally robust and easy to handle without
damaging them.
Page 3 of 23
SECTION 1: CONTINUED
Torpedo
shaped
Anax spp
Aeshna spp
Brachytron
Long
body
with
rectangu
lar head
Cordulegaster
boltonii
Robust
with
relatively
long legs
C. aenea
Somatochlora
spp
Robust
with
relatively
short
legs
Libellula spp
O.
cancellatum
Small,
with
relatively
long legs
Sympetrum
spp
L. dubia
Small
with
relatively
short
legs
O.
coerulescens
Triangul
ar head,
with
thick
antennae,
relatively
short
legs.
Gomphus
vulgatissimus
Cordulegastridae
There is only one species of cordulegastrid in Britain, Cordulegaster
boltonii. This species usually breeds in small streams in Britain. The deep
crenulations on the front of the mask and long narrow body with rectangular
head are characteristic and the larvae can be identified to species level in
any larval stage
Body long and hairy
Mask with deep crenulations
Page 4 of 23
Gomphidae
The only species of gomphid in Britain is Gomphus vulgatissimus. The
species is restricted to large lowland rivers in England and Wales. The
larvae have a distinctive flattish shape when viewed from the side and a
characteristic triangular-shaped head and broad antennae. It can be
identified to species level in any larval stage.
Head with triangular shape,
antennae with four thick
segments
Body flattish in side profile
Aeshnidae
There are eight species of aeshnid in three genera. The three genera,
Brachytron, Anax and Aeshna can be distinguished from each other by
overall body shape and the shape of the head.
Head with very
small eyes
Head with eyes
broader than long
Rounded head with
eyes as long as
broad
Brachytron pratense
Aeshna sp
Anax sp
Page 5 of 23
Brachytron has small eyes, a tapered head and long lateral spines on the
abdominal segments. The only British species is B. pratense. With practice
this species can be identified to genus from the head shape in any larval
stage.
Anax has a rounded head and large eyes, the hind margins of which are
relatively straight and parallel to the hind margin of the head. The eyes are
typically longer than they are broad. The most abundant resident species in
Britain at present is A. imperator. The migrant species A. parthenope has
begun to appear regularly in this country in the last 10 years and there are
now records of the species breeding. For this reason it seems appropriate
to include both species in this module. They are rather tricky to separate
and the most reliable way is to examine the relative length of the male
projection, which lies over the base of the epiproct (the central one of the
five appendages at the tip of the abdomen). In A. imperator the male
projection is as long as broad, but in A. parthenope the male projection is
about half as long as broad. With practice both species can be identified to
genus from the head shape in any larval stage.
Male projection is as long as
broad
Male projection is about half as
long as broad
Anax imperator
Anax parthenope
Page 6 of 23
Aeshna larvae have large eyes and a tapering head. The hind margins of
the eyes are not as straight or parallel to the hind margin of the head as in
Anax sp. The eyes are typically broader than they are long compared to
Anax sp. The overall size and the shape and relative dimensions of the
labium and the lateral spines on the abdominal segments can distinguish
the species. The shape of the labium should be determined by measuring
the ratio between the width of the labium at its narrowest part to the length
of the prementum from the basal hinge to the front of the hooks. The ratio
between these two measurements will narrow down the options for
identification.
The two smallest Aeshna larvae are A. mixta and A. caerulea and both are
shorter than 38 mm when in the final stadium. In A. mixta the lateral spine
on abdominal segment 10 almost reaches the apex of the segment, but in
A. caerulea the lateral spine on abdominal segment 10 is shorter and
reaches only about one-third of the length of the segment. A. caerulea is
known only from Scotland but A. mixta occurs throughout much of England
and Wales and has recently spread north into southern Scotland.
The larger Aeshna species are best distinguished by the shape of the
labium and comparison of the length and distribution of lateral abdominal
spines. The labium in A. cyanea looks noticeably narrow and is at least
twice as long as broad. In A. grandis and A. juncea the labium is much
broader. A note of caution: the labium in A. mixta is also narrow, so check
the size of your specimen and make sure that it is in the final larval stadium.
Take a look at the lateral abdominal spines to sort out A. grandis from A.
juncea. A well-developed lateral spine is present in larvae of A. grandis but
is very small in A. juncea. In most specimens of A.grandis a distinctive
cream strip is present on the side of the thorax and head. A note of
caution: A. caerulea could be confused with A. juncea so check the size of
your specimen and make sure it is in the final larval stadium.
Page 7 of 23
Labium broad
Aeshna grandis
A. juncea
A. isosceles
Lateral spines on S9 short
Aeshna juncea
Labium long and narrow
Aeshna cyanea
A. mixta
A. caerulea
Lateral spines on S9 long
Aeshna mixta
A. grandis
A. isosceles
Aeshna isosceles is a rare species restricted to Norfolk and Suffolk where it
favours ditches filled with Water Soldier (Stratiotes aloides) on grazing
marshes. The species is protected under the Wildlife and Countryside Act
so a license from Natural England is required to handle the larvae. Larvae
are similar to other large aeshnids and can be distinguished by measuring
the anal appendages. The two cerci are especially long relative to the
epiprocts.
Page 8 of 23
EXERCISE 1: EXAMINE THE LABIUM AND
ABDOMINAL SPINES OF AN AESHNID LARVA
This mandatory exercise is worth 10 points.
Using a pair of fine forceps, pull out the mask in front of the head of an
aeshnid larva. Attach a photograph or make a sketch of the labium and
note the overall shape of the mask. Also examine the lateral spines on the
abdomen and attach a photograph or a sketch, which shows the length of
these spines relative to the length of each abdominal segment. Write
down which species you think it is.
Complete exercise 1 in the space below or use the template appended to
the introductory leaflet.
Mark =
/ 10
Page 9 of 23
Corduliidae
There are three British species of Corduliidae in two genera, Cordulia and
Somatochlora. Corduliids can be readily distinguished from similar-sized
Libellula and Orthetrum by the relatively longer legs. This is best assessed
by looking at the relative position of the tibial-tarsal joint compared to the
tip of the abdomen (see below). C. aenea has a broad black band on the
shoulders of the mesothorax and pale dorso-lateral spots on the abdomen.
S. metallica has black dorso-lateral spots on the abdomen. This species
also has much larger mid-dorsal abdominal spines than C. aenea, although
in some specimens these spines may be obscured by debris. S. arctica is
very distinctive because it is covered in hairs, which are lacking in the other
two British cordulids and all other dragonflies.
Robust with long legs, tibialtarsal joint extends beyond tip of
abdomen
Robust with short legs, tibialtarsal joint does not extend
beyond tip of abdomen
Cordulia aenea
Libellula sp
Somatochlora sp
Orthetrum sp
Libellulidae
There are nine species of Libelullidae resident in Britain in four genera,
Libellula, Orthetrum, Sympetrum and Leucorrhinia. There are an additional
two migrant species of Sympetrum that occasionally breed in this country.
The larvae of Libellula and Orthetrum are robust, with square heads, small
eyes and short legs. The larvae of Sympetrum and Leucorrhinia, on the
other hand, are smaller (Note of Caution: O. coerulescens is also small but
has short legs), have tapering heads with large eyes, and the legs are
longer. Leg length can quickly be determined by assessing whether the
position of tibial-tarsal joint extends beyond the abdomen. Looking at the
larva head-on, the mask reaches the base of the eyes in Sympetrum and
Leucorrhinia species but not in species of Libellula and Orthetrum.
Page 10 of 23
Species of Libellula have a dorsal spine on abdominal segment 8, but this
spine is absent in species of Orthetrum. L. fulva has very distinctive middorsal abdominal spines, which are much larger than the other two Libellula
species. L. fulva is restricted to lowland English rivers.
An examination of the depth of the serrations dividing the labial palps on
the front of the mask will distinguish L. depressa in which the serrations on
the mask are deep, whereas they are relatively shallow in the other two
Libellula species.
Labial palps with deep
serrations
Labial palps with shallow
serrations
Libellula depressa
NB. Cordulids also show this feature but
have much longer legs
Libellula quadrimaculata
A useful field character for larvae of L. depressa and L. quadrimaculata is a
yellow fleck present on the front-side of labium. This marking is absent in
other libellulids and cordulids.
The two Orthetrum species are best distinguished from each other by size.
Final stadium larvae of O. cancellatum are greater than 23 mm in length
whereas O. coerulescens larvae are less than 23 mm in the final stage.
There is a small mid-dorsal spine present on abdominal segment 7 in O.
coerulescens, which is absent in O. cancellatum, but this spine is
sometimes difficult to see. O. coerulescens can be abundant in bog pools.
The larvae of Sympetrum and Leucorrhinia are small and have tapering
heads with large eyes, and long legs. Larvae of Leucorrhinia dubia are
characterised by three dark longitudinal stripes on the underside of the
abdomen. This species is usually restricted in bog pools.
Page 11 of 23
Sympetrum species are best separated by checking the length of the
abdominal dorsal and lateral spines. The only British Sympetrum with no
mid-dorsal abdominal spines is S. fonscolombii, a migrant species that only
occasionally breeds in Britain. This species is also marked with two dark
dorso-lateral stripes on the abdomen.
If dorsal spines are present then check the relative lengths of the lateral
spines paying particular attention to that on S9. If the lateral spines are long
and the one on segment 9 is more than half the length of segment 9, then
the species will be either S. striolatum or S. sanguineum. These two species
can be tricky to separate but in most specimens if the lateral spine is longer
than segment 9 then it will be S. striolatum whereas in S. sanguineum the
lateral spine on segment 9 is usually shorter than the length of the segment.
Lateral spine on S9 short
Lateral spine on S9 long
S. danae
S. striolatum
S. flaveolum
S. sanguineum
The two species with relatively short lateral abdominal spines are S. danae
and S. flaveolum. S. flaveolum is a migrant species to Britain and breeding
colonies are rare in Britain at present, although this may change if climate
warming continues. It has a mid-dorsal spine on abdominal segment 8
which is absent in S. danae. The lateral spine on abdominal segment 9 is
about half the length of abdominal segment 9 whereas in S. danae the
lateral spine on this segment is considerably less than half the length of the
segment.
EXERCISE 2: IDENTIFY A PRESERVED
ANISOPTERAN LARVA
Page 12 of 23
Find a preserved anisopteran larva of any family. In the space below either
attach a photograph (more than one if necessary) or make a sketch of your
specimen. The sketch does not need to be of the whole animal, just the
diagnostic features. Write down which species you think it is. Also write
down why you think it is this species (your ‘diagnosis’). Include arrows
pointing to the characteristic parts if you feel it necessary to do this.
Mark =
/ 10
Page 13 of 23
SECTION 2: DAMSELFLIES (ZYGOPTERA)
For this section you will need a copy of the British Dragonfly Society Field
guide to the larvae and exuviae of British dragonflies. Volume 2. Damselflies
(Zygoptera). (Cham, S. in press).
The larvae of Zygoptera possess three large and distinctive paddle-shaped
caudal lamellae at the tip of the abdomen. In general only larvae in the final
stage can be reliably identified to species level. Some species have
characteristic features in all larval stages, which enable identification to
genus. Larvae in the final stadium can be recognised by the length of the
wing buds, which reach the fourth abdominal segment. You should not
attempt to identify to species level, any larvae in which the wing buds are
shorter than this unless specifically advised. Many of the features that
characterise each species are not fully developed. All damselfly larvae have
ten abdominal segments. The first abdominal segment is often rather
difficult to see so it is best to count from the last segment backwards.
The most important features that you will need to examine in order to
identify most zygopteran larvae are:
• the shape of the head;
• the shape of the mask (labium)
• the number of setae present on the mask
• the shape and markings of the caudal lamellae.
Page 14 of 23
Note: Zygoptera larvae easily shed the caudal lamellae, which then re-grow
as the larva develops. For this reason the number and shape of the caudal
lamellae can vary. If the caudal lamellae do vary then only the largest one
should be used when identifying a specimen.
The number of setae on the mask can be examined by gently pulling the
mask forward in front of the head with a pair of forceps.
The larvae of Zygoptera are generally robust and easy to handle without
damaging them.
Stick-like’ with
rigid caudal
lamellae. ‘Hornlike’ antennae with
1st segment as
long as all others
combined.
Calopterygidae
Large with dark
banded caudal
lamellae. Labium
long and ‘racketshaped’.
Larvae have
distinctive
blotching on
caudal lamellae
that terminate with
a thin filament at
the tip.
All others
Lestidae
Platycnemididae
Coenagrionidae
Caution: E. najas
can be similar in
size and has banded
lamellae.
Page 15 of 23
Calopterygidae
There are two species of Calopterygidae in Britain and they can be
distinguished by the markings on the caudal lamellae, but the most reliable
feature is the presence or absence of a stout spine on the back of the head.
In Calopteryx virgo this spine is prominent but in C. splendens the spine is
inconspicuous. Additionally, the caudal lamellae of C. virgo are marked with
a single pale vertical band, whereas in C. splendens the caudal lamellae are
usually marked with two pale vertical bands. Both species are largely
restricted to flowing water. C. virgo occurs in gravel-bottomed, rapidly
flowing streams and C. splendens is found in more sluggish streams and
rivers with a muddy bed, and occasionally standing waters.
Prominent pointed spine behind eyes
Rounded spine behind eyes
Calopteryx virgo
Calopteryx splendens
Early instar larvae are distinctive and can be readily identified to genus.
Lestidae
There are two resident species of lestid in Britain, which both have a
distinctive tennis racquet-shaped labium. The species can be distinguished
from each other by the shape of the caudal lamellae. In Lestes sponsa the
caudal lamellae are parallel-sided, whereas in L. dryas the caudal lamellae
distinctly taper towards the tip from about the mid-point. Female
specimens can also be distinguished by the relative length of the ovipositor,
which in L.dryas extends beyond the tip of the abdomen but is shorter in L.
sponsa. L. dryas is currently restricted to the marshes of the Thames
Estuary and East Anglia in England (although it is more widespread in
Ireland) where it occurs in densely vegetated ditches and pools which often
dry-out in the late summer.
Page 16 of 23
Caudal lamellae parallel sided
Caudal lamellae tapering
Lestes sponsa
Lestes dryas
Platycnemididae
Platycnemis pennipes is the only species in this family currently resident in
Britain and is largely restricted to lowland rivers and canals.
Caudal lamellae with dark blotches terminating in long filament. Hind margin of
head rectangular. Legs banded.
Platycnemis pennipes
Page 17 of 23
Coenagrionidae
This is by far the largest family of Zygoptera in Britain. Currently there are
12 resident species divided between six genera: Pyrrhosoma, Ceriagrion,
Coenagrion, Enallagma, Ischnura and Erythromma. Most can be
distinguished by the shape and markings on the caudal lamellae, but
markings on the head and legs, and the shape of the head are also a useful
guide.
The larvae of both Pyrrhosoma nymphula and Ceriagrion tenellum have
rather broad, rectangular heads whereas the other coenagrionid species
have narrow heads that taper from behind the eyes. Living larvae of P.
nymphula have a distinctive posture in which the tip of the abdomen is
flexed upwards. The caudal lamellae are also distinctive, being broad,
pointed at the tip and marked with heavy, dark blotches, often in the shape
of a X. In contrast, larvae of C. tenellum have rather small, broader lamellae
with faint blotches. Towards the tip of the lamellae, setae are absent from
the margin whereas in P. nymphula fine setae are present along the margin
near the tip. A useful confirmatory character, which requires the labium to
be pulled out in front of the head, is that only a single pair of setae is
present on the labium in C. tenellum, whereas in P. nymphula there are three
pairs of setae in this position. C. tenellum is largely restricted to shallow
pools in bogs and seepages.
Hind margin of head rectangular
Hind margin of head rounded
Coenagrion spp
Pyrrhosoma nymphula
Ischnura spp
Ceriagrion tenellum
Erythromma spp
Enallagma cyathigerum
Page 18 of 23
Caudal lamellae with dark X-like
markings
Caudal lamellae with faint
blotches
Pyrrhosoma nymphula
Ceriagrion tenellum
The two species of Erythromma can be distinguished from other
coenagrionid species by the presence of rows of coarse hairs or setae on
the underside of the abdomen. Such setae are absent in other
coenagrionids. E. najas is larger than other coenagrionid species in the final
larval stage and has distinctive dark vertical bands on the caudal lamellae,
which are reminiscent of Lestidae, but the dark bands are restricted to the
apical half of the lamellae. The labium is broad in E.najas, which differs from
the tennis racquet-shape of Lestes. In E. viridulum the banding on the
caudal lamellae is absent, although they may be mottled.
Underside of S2 and S3 with rows of setae
Erythromma spp
Page 19 of 23
The remaining species of Coenagrionidae are a little more tricky to separate
from each other, but the presence or absence of spots on the back of the
head and bands on the legs, together with the shape of the caudal lamellae
can be a useful guide.
Enallagma cyathigerum has no spots on the back of the head. It has rather
broad caudal lamellae, which are marked with 1-3 narrow dark vertical
bands, and are not nodate (i.e. there is no junction between the basal and
apical halves, which are of similar thickness). There are six antennal
segments.
Hind margin of head spotted
Hind margin of head un-spotted
Coenagrion puella
Ischnura elegans
Coenagrion pulchellum
Ischnura pumilio
Enallagma cyathigerum
Ischnura species have rather long, narrow and pointed caudal lamellae. I.
elegans has conspicuously banded legs, whereas in I. pumilio the bands on
the legs are absent or very faint. In I. elegans the hairs on the lateral ridges
of the abdominal segments are stouter than other hairs on the abdomen
whereas in I. pumilio the hairs on the lateral ridges of the abdominal
segments are the same size as other hairs. These hairs are best viewed
under a microscope using backlighting. I. pumilio has a restricted
distribution in England where it occurs in shallow seepages and pools,
which are often temporary.
Page 20 of 23
The point at which the coarse setae end on each margin of the caudal
lamellae can help to separate the common small damselflies (arrowed in
table below) and are a useful supplement to other characters.
Node not offset
Coenagrion puella
C. pulchellum
(similar but more
rounded at tip)
Narrow lamellae with
offset node and very
pointed tip
Ischnura elegans
Broad with vertical
lines
Enallagma
cyathigerum
Most of the species of Coenagrion have spots on the back of the head and
nodate caudal lamellae (i.e. the lamellae are thicker in the basal half than
the apical half and there is a distinct junction between the two halves). C.
pulchellum and C. puella have seven antennal segments and both species
have a node on the caudal lamellae that is similar to a sigmoid curve.
However these two species cannot be reliably separated from each other.
C. lunulatum is an Irish species, which has six antennal segments and five
setae on the labial palp (the hinged part of the mask). C. hastulatum is a
Scottish species, which also has six antennal segments but only four setae
on the labial palp. This species has a distinctive node that is perpendicular
to the trachea and forming a distinctive joint. C. mercuriale is the only
Coenagrion species, which has no node on the caudal lamellae and lacks
spots on the back of the head. The species is distinctive in being smaller
than the other Coenagrion species and having rather short, narrow,
unmarked lamellae, which are distinctly hairy at the tip, being covered in
fine setae. It has a seven-segmented antenna. The species has a restricted
distribution in England and Wales where it occurs in shallow streams and
seepages.
NB. A license from Natural England is required before handling larvae of this
species.
Page 21 of 23
EXERCISE 3: EXAMINE THE LABIUM AND
CAUDAL LAMELLA OF A COENAGRIONID LARVA.
Using a pair of fine forceps, pull out the mask in front of the head of a
coenagrionid larva. Attach a photograph or make a sketch of the labium
and note the number of setae arranged in two patches on the pre-mentum
(the main part of the labium) and the number of setae present on the labial
palps, the hinged part of the mask in front of the pre-mentum.
Carefully detach the central caudal lamella from a zygopteran larva using a
pair of fine forceps. Attach a photograph or make a sketch of the lamella
noting whether or not the lamella is nodate, indicating any markings on the
lamella and also the distribution of setae along the margin.
Write down which species you think it is.
Mark =
/ 10
EXERCISE 4: IDENTIFY A PRESERVED
ZYGOPTERAN LARVA
Page 22 of 23
Find a preserved zygopteran larva of any family. In the space below either
attach a photograph (more than one if necessary) or make a sketch of your
specimen. The sketch does not need to be of the whole animal, just the
diagnostic features. Write down which species you think it is. Also write
down why you think it is this species (your ‘diagnosis’). Include arrows
pointing to the characteristic parts if you feel it necessary to do this.
Mark =
/ 10
Page 23 of 23
Hooray - you have finished! Your final mark is written in the box below.
Mark:
/ 40 =
%: Pass
/
Fail
Recording the occurrence of Dragonflies and Damselflies is of primary
importance in enhancing our understanding of these fascinating insects and
taking significant steps towards their protection and the conservation of
their habitats. Within the British Dragonfly Society, the Dragonfly Recording
Network (DRN) has been set up to administer Odonata records in the British
Isles. Records of larvae are especially important as they indicate whether a
species is successfully breeding at a site.
For more information on the Dragonfly Recording Network contact:
Steve Prentice
c/o Natural England (West Midlands)
Attingham Park
Shrewsbury
SY4 4TW
phone - 01743 282 019
e-mail - [email protected].
or visit the website at:
http://www.dragonflysoc.org.uk/recording.html

Dragonflies and damselflies

Transcription

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