ホシザキグリーン財団研究報告特別号 第32号: 51 ― 62 頁，2023 年 1 月
Spec. Bull. Hoshizaki Green Found.（32）: 51 ― 62，Jan. 2023

Life History of an Assassin Bug, Sirthenea flavipes（Stål, 1855）:
Laboratory Rearing and Field Observations＊
Masakazu HAYASHI
Hoshizaki Green Foundation, Sono 1664 ― 2, Izumo, Shimane Pref., 691 ― 0076 Japan
Abstract

A female adult of Sirthenea flavipes（Stål, 1855）collected in June 2022

from Izumo City, Shimane Prefecture, Japan, laid over 30 eggs under laboratory
conditions. The eggs hatched after approximately two weeks in the laboratory. The
first-instar nymphs hatched in early July and reached the fifth（last）instar in midAugust. The S. flavipes nymphs preyed on the nymphs of mole crickets（Gryllotalpa
orientalis Burmeister, 1839）. Each nymph instar could be determined based onsize, wing bud shape, and head coloration. Early instars of S. flavipes appearedin the field from July, concomitantly with the appearance of early instars of molecricket nymphs; mature fifth-instar nymphs of S. flavipes were observed in thefield in late August; and the emergence of adult S. flavipes was confirmed in thelaboratory in early September. Thirty S. flavipes nymphs were collected between
July 13 and September 15, 2022. Sirthenea flavipes overwinters as adults, but not as
nymphs. The S. flavipes nymphs were similar in body coloration and body size（morespecifically, the second- and third-instar nymphs）to those of the toxic Paederus
fuscipes Curtis, 1826, suggesting a possible case of mimicry.
Key words：biotope, Hemiptera, Reduviidae, wetlands, Paederus fuscipes

Introduction
Sirthenea flavipes（Stål, 1855）is distributed in Japan（Honshu, Shikoku, Kyushu, Nansei
Islands）, Taiwan, and the Oriental Region, is 18 ― 20 mm long, inhabits wet ground, and often flies
toward lights（Yasunaga et al., 1993）. The body color of the adult is yellow, with black coloration on
the posterior edge of the prothorax, scutellum, and forewings（Yasunaga et al., 1993）. In contrast,
the nymphs are reddish orange, and their head, scutellum, and wing buds are black（Ishikawa etal., 2012）. The genus Sirthenea, including S. flavipes, is a known predator of mole crickets. Hudson（1987）examined nymphs and adults of a New World species, Sirthenea carinata（Fabricius, 1798）,
obtained from the field and laboratory-rearing, and reported the existence of five nymph instars andthat the first three instars prefer crickets, while the fourth and fifth-instars and adults prey on molecrickets. To our knowledge, there are no literature records of S. flavipes feeding in Japan; however,images of adult S. flavipes feeding on mole crickets（Gryllotalpa orientalis Burmeister, 1839）have
been posted on the Internet. In addition, the similarity of the dorsal pattern of S. flavipes to that of abombardier beetle, Pheropsophus occipitalis jessoensis Morawitz, 1862 and the fact that both species＊ホシザキグリーン財団研究業績

第 373 号

51

Masakazu HAYASHI

地理院地図(電子国土Web)例：剱岳

金沢市木ノ新保町

度 分 秒

度 分 秒

初期表示

印刷

共有

設定

ツール

? ヘルプ

地図

Lake Shinji-ko
宍道湖

Study site

あ度

島根県出雲市園町（付近の住所。正確な所属を示すとは限らない。）
分

ポイント

秒

度

分

秒

ズーム

Light trap

Fig. 1. Study site and location of thelight trap at the Hoshizaki Institute for
Wildlife Protection, Izumo. Modified froma GSI map（https://maps.gsi.go.jp）.

100 mprey on mole crickets have led to increased interest in the biology of this species.
Adults of S. flavipes are often seen flying toward lights, while nymphs are rarely observed. Thus,
both adults and nymphs are expected to be underground rather than aboveground in nature. The
present author observed mole crickets and S. flavipes at a waterside of an artificial wetland in Izumo
City, Shimane Prefecture, Japan, in April 2022, and thus investigated the habitat of S. flavipes in
captivity and in the field. Little information is available in the literature on the ecology of S. flavipes
from Japan. The information obtained from this study is therefore summarized in the present paper
and the estimated life history of S. flavipes is reported.

Methods and materials
1. Field observation
The study was conducted near the waterʼs edge of an artificial wetland in Sono-cho, Izumo
City, Shimane Prefecture, Japan（35.4481°N, 132.8641°E; alt. 0.5 m; Fig. 1）. The water level inthe wetland（Fig. 2a ― d）was regulated and maintained at maximum level（20 ― 30 cm deep）from
mid-April to late August. The aquatic plants Phragmites australis and Zizania latifolia grew near
the waterʼs edge. Insect surveys were conducted 34 times from April 11 to September 22, 2022 by
digging in the riparian mud and soil for 30 ― 40 minutes using a hoe（Fig. 2e ― k）. The collected
nymphs were preserved in 70 % ethanol and some were used for laboratory rearing.
2. Light trap
Hoshizaki Institute for Wildlife Protection is adjacent to the artificial wetland. White and black
fluorescent lights outside of the Institute were turned on at night, and two buckets with funnels
were installed to capture flying insects（35.447991°N, 132.866115°E; alt. 0.5 m; Fig. 1）. This was
performed from June 27 to October 8, 2021 and June 20 to September 29, 2022, except on days ofinclement weather or because of other factors.
3. Laboratory rearing
One adult female from soil（June 20, 2022）and one adult female from a light trap（June 22,2022）were collected and placed in 500-ml cylindrical containers（translucent）with moistened
paper（Kimwipes, Nippon Paper Crecia Co., Ltd.）for rearing（Fig. 3a）. Mole crickets obtained
from the study site were used as food（Fig. 2f, i）. The females were kept in the laboratory, with no52

Life History of an Assassin Bug, Sirthenea flavipes（Stål, 1855）: Laboratory Rearing and Field Observations

Fig. 2. Study site in 2022. a ― d, artificial wetland in Sono-cho, Izumo City（a, April 11; b, June 6; c, July 31;
d, September 15）; e, riparian mud; f, mature nymphs and adults of mole crickets（April 18）; g, female adult ofSirthenea flavipes; h, soft mud of waterʼs edge（July 13）; i, first nymphs of mole crickets collected from soft mud（July 1）; j, third nymph（August 11）; k, fifth nymph on water surface（August 19）.

53

Masakazu HAYASHI

temperature or light/dark control. Room temperature was recorded with a data logger. Eggs foundon the moistened paper were placed in the same cylindrical container and observed.
Hatched nymphs were reared in groups in a 500-ml plastic container（129×99×60 mm, L×W×
H; translucent）set with moistened paper（Fig. 3b）. Some nymphs were reared on the small cricket,
Pteronemobius ohmachii（Shiraki, 1930）. As live crickets were not preyed on, fresh dead crickets
were used.
4. Photographs
Photographs of the immature stages of S. flavipes were taken using a Canon EOS 90D（Canon
Inc., Tokyo, Japan）with a macro lens（MP-E 65 mm; Canon Inc.）at equal magnification.
Photographs and movies in the field were taken using an iPhone SE（Apple Inc., Cupertino, CA,USA）and edited using iMovie version 10.3（Apple, Inc.）.
5. Measurements
The body parts of S. flavipes were measured. The width of the head（between the two ends ofthe compound eye）and the maximum width of the prothorax were measured from images taken at
equal magnification with a macro lens and Canon EOS 90D.

Fig. 3. Containers used for laboratory rearing. a, 500-ml cylindrical containers with moistened paper for therearing of adults and eggs; b, c, 500-ml plastic container with moistened paper for the rearing of nymphs（b, withlive mole crickets; c, with small dead crickets）.

54

Life History of an Assassin Bug, Sirthenea flavipes（Stål, 1855）: Laboratory Rearing and Field Observations

Results and discussion
1. Field observation
Female adults of S. flavipes were collected from mud and soil on May 19 and June 20, 2022（Fig.
2g）. Thirty nymphs were collected between July 13 and September 22, 2022（Table 1）. The nymphs
were found in soft mud and soil at the waterside（Fig. 2h, j）. Early instars of nymphs were found invery shallow mud, especially at the waterʼs edge. Early instars of mole crickets were also found on
the same dates. The fifth-instar nymph collected on August 26 hatched on September 5, and fifthinstar nymphs collected on September 4 hatched on September 10（Fig. 5e）. The abdomens ofmature fifth-instar nymphs were significantly swollen. Nymphs were able to move quickly over thewater surface（Fig. 2k）.
Overwintered adults and nymphs of mole crickets were found from April to July（Fig. 2f）; early
instars of mole crickets appeared in riparian soft mud from July 1, 2022（Fig. 2i）.
Other organisms found at the waterʼs edge at the study site included Procambarus clarkii（Girard,1852）, Hydroglyphus japonicus（Sharp, 1873）, Paederus fuscipes Curtis, 1826, Ochthera circularis
Cresson, 1926, Fejervarya kawamurai Djong, Matsui, Kuramoto, Nishioka et Sumida, 2011,
Pelophylax nigromaculatus（Hallowell, 1861）, among others.
Table 1. Number of Sirthenea flavipes collected from the study site in 2022
Month Apr
Day

May

Jun

Jul

11 15 18 20 27 18 19 30 6 15 20 1

Aug
7 10 13 21 22 25 27 31 1

1st

2

2nd

2

3

3rd

2

2

8 11 12 19 23 26 29 4

1

4th

2

1

9 15 22

1

1

1

1

2

1
1
1

1

7

1

1

5th
Adult

Sep
4

1

1
1

1
1

1

2. Light trap
A total of 15 and 20 S. flavipes adults were captured in 2021 and 2022, respectively; in 2021 they
did not fly before mid-August, but in 2022 they flew intermittently from June through September.
The flight of adults toward lights is highly influenced by weather and widely varies each year;
individuals flying after late August are considered to be new adults because of their brightcoloration.
Records. 1 ex., Aug. 23, 2021; 1 ex., Sep. 8, 2021; 2 exs., Sep. 11, 2021; 1 ex., Sep. 12, 2021; 1 ex.,Sep. 16, 2021; 1 ex., Sep. 27, 2021; 4 exs., Sep. 28, 2021; 2 exs., Sep. 29, 2021; 1 ex., Sep. 30, 2021; 1
ex., Jun. 22, 2022; 1 ex., Jul. 16, 2022; 1 ex., Aug. 1, 2022; 1 ex., Aug. 2, 2022; 2 exs., Aug. 5, 2022; 3
exs., Aug. 8, 2022; 1 ex., Aug. 13, 2022; 1 ex., Aug. 19, 2022; 2 exs., Sep. 5, 2022; 1 ex., Sep. 10, 2022;
2 exs., Sep. 12, 2022; 2 exs., Sep. 14, 2022; 2exs., Sep. 18, 2022; 1ex., Sep. 23, 2022; 2 exs., Sep. 25,
2022; 1 ex., Sep. 29, 2022.
3. Laboratory rearing
One female collected from the study site on June 20, 2022, was reared and began laying eggs onJune 22. Eggs were laid sporadically on moistened paper in the container. The first egg hatched on
55Masakazu HAYASHI

July 4（Fig. 4a ― h）. The female laid at least 30 eggs. In addition, a female collected by light trap on
June 22 also laid eggs.
Fifteen laboratory-hatched first-instar nymphs（Fig. 4i, j）were reared in groups in a 500-ml plastic
container and fed with early instars of mole cricket nymphs（body size: 5 ― 6 mm）, and predation
was observed. The nymphs that attacked and fed on living mole crickets showed marked abdominal

Fig. 4. Eggs and nymphs of Sirthenea flavipes. a ― d, egg（a, June 26; b, June 30; c, July 1; d, July 4）;
e ― h, hatching nymph; I, j, first-instar nymph; k, l, second-instar nymph; m, n, third-instar nymph. Scalebars＝5.0 mm.

56Life History of an Assassin Bug, Sirthenea flavipes（Stål, 1855）: Laboratory Rearing and Field Observations

swelling. Second-, third-, and fourth-instar nymphs of S. flavipes first emerged on July 16（Fig. 4k, l）,
July 20（Fig. 4m, n）, and July 27（Fig. 5a, b）, respectively. On August 7 ― 8, the mature fourth-instar
nymphs began attacking one other, and eight third-instar nymphs and two fourth-instar nymphs
died. The remaining three fourth-instar nymphs were kept separately. One fourth-instar nymph diedon August 23. One fifth-instar nymph（Fig. 5c, d）emerged on August 18. The remaining two fifthinstar nymphs died on August 31 and September 12, respectively, and did not hatch.
Eight nymphs were reared in another 500-ml plastic container with small crickets. They did notprey on live crickets, but only sucked dead crickets（Fig. 5f）. Only two grew to third-instar nymphs,
and died before reaching the fourth-instar.

Fig. 5. Nymphs of Sirthenea flavipes. a, b, fourth-instar nymph; c, d, fifth-instar nymph; e, newly emerged
adult; f, second-instar nymph feeding on small dead crickets. Scale bars＝5.0 mm（except for 5e）.

57

Masakazu HAYASHI

4. Size and morphological changes of nymphs
Nymph body was reddish orange and differed from that of adults; the thorax and abdomen did
not change from the first to fifth instars, but the head changed to reddish orange in the fifth instar（Fig. 6）. In the fourth instar, the wing buds of the forewings elongated and became almost as longas the thorax（Fig. 6d）, and in the fifth instar, the wing buds of the forewings further elongated andbecame distinctly longer than the thorax（Fig. 6e）.
Measurements of nymphs are shown in Table 2 and Fig. 7. The measurement of 11 first-instar,four second-instar, 12 third-instar, seven fourth-instar, and six fifth-instar nymphs confirmed that the
instars can be distinguished based on the head width and prothorax width.
Key to instars of S. flavipes:
1a. Posterior edge of the prothorax developed and black colored. Forewing covering abdomen …

adult

1b Posterior edge of the prothorax not developed and orange colored. Forewing not covering
abdomen … 2（nymphs）

2a Head orange colored. Wing buds of the forewings longer than prothorax … fifth instar
2b Head entirely black … 3

3a Wing buds of the forewings as long as prothorax … fourth instar
3b Wing buds of the forewings without growth … 44a Maximum width of prothorax greater than 1.5 mm and less than 2.0 mm … third instar
4b Maximum width of prothorax less than 1.5 mm … 55a Maximum width of pronotum greater than 1.2 mm and less than 1.5 mm … second instar
5b Maximum width of pronotum less than 1.1 mm … first instar

Fig. 6. Nymphs of Sirthenea flavipes. a, first instar; b, second instar; c, third instar; d, fourth instar;e, fifth instar. Scale bar＝5.0 mm.

58

Life History of an Assassin Bug, Sirthenea flavipes（Stål, 1855）: Laboratory Rearing and Field Observations

collected from study site
reared nymphs

(mm)
4.0

Maximum width of prothorax

Width of prothorax
Width of head

5th

3.0

Width of Head

4th

2.0
3rd
2nd

1.0
1st

0

1.0

2.0
Width of head(mm)

Fig. 7. Scatterplot of the head width versus the prothorax width of nymphs
Table 2. Size of nymphs of Sirthenea flavipes
Instars of nymph

1st

2nd

3rd

4th

5th

Width of head

0.62 ― 0.69

0.83 ― 0.90

1.07 ― 1.18

1.38 ― 1.49

1.75 ― 2.01

（mean）mm

0.66

0.86

1.14

1.43

1.88

0.79 ― 0.98

1.24 ― 1.34

1.65 ― 1.93

2.13 ― 2.48

2.71 ― 3.43

（mean）mm

0.87

1.30

1.80

2.29

3.13

N of specimens

11

4

12

7

6

Width of prothorax

5. Life history
The comparison of the life history of S. flavipes with that of the mole crickets is shown in Fig. 8.
The emergence period of new adults was considered to be when brightly colored adults flew toward
the lights, and the last-instar nymphs were considered to have hatched in the field after August.
These new adults overwintered without reproducing. As the adults live underground, monitoring
their activity in the field was difficult; nevertheless, their activity was resumed in May at the latest.
The egg-laying season was from June to August, and the eggs may have been laid sporadically increvices in the soil, such as mole cricketsʼ tunnels. Eggs hatched after approximately two weeks.
In the study site, early instars of mole crickets were abundant in the soft mud at the waterʼs edge59

Masakazu HAYASHI

in July and August, and first- to third-instar nymphs of S. flavipes were seen at the same time,suggesting that they come to the waterʼs edge to prey on early instars of mole crickets. The fifthinstar nymphs of S. flavipes matured in late August, and new adults emerged in September or later.
It is unclear how long the nymphsʼ emergence period lasts after September, but the absence ofnymphs from April to May suggests that the nymphs do not overwinter as nymphs.
At the study site, approximately half of the mole crickets observed in April and May were nymphs,
suggesting that most nymphs that hatched the previous year were wintering before reaching oldage. Although the egg-laying season for mole crickets is considered to be from May onward（Akino
et al., 1956）, first-instar nymphs were not seen at the study site until July, and the egg-laying season
was later, from June onward, which may be related to the large number of overwintering nymphs（Fig. 8）.

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Overwintering/overwintered adults
Eggs
Nymphs?
New adults

Sirthenea flavipes

Adults collected by light traps

Overwintering/overwintered nymphs and adults
Eggs
Nymphs

Gryllotalpa orientalis

Immature nymphs congregate insoft mud at the water's edge
New adults

Fig. 8. Estimated life histories of Sirthenea flavipes and Gryllotalpa orientalis in Izumo City,
Shimane Prefecture, Japan

60

Life History of an Assassin Bug, Sirthenea flavipes（Stål, 1855）: Laboratory Rearing and Field Observations

Fig. 9. Adult of Paederus fuscipes

6. Nymph behavior and possible mimicry
Early instars of S. flavipes nymphs are often found in the same very shallow mud at the waterʼsedge as early instars of mole crickets during the daytime. Therefore, they were often found with
only a little digging in the mud. Sirthenea flavipes nymphs that appeared on the surface walked
quickly or walked on the surface of the water, and often hid in mud crevices on the landward side.
Sirthenea flavipes nymphs are reddish orange with black markings on the head, mesothorax,
dorsal surface of the abdomen, antennae, and legs. This coloration is similar to that of Paederus
fuscipes（Staphylinidae; rove beetles: Fig. 9）, which is 6.5 ― 7.5 mm in length（Kurosa, 1958）,
approximately the same size as second-instar nymphs of S. flavipes with a full-grown abdomen or
third-instar nymphs with an undersized abdomen. At the study site, the beetles were frequently
observed roaming the ground near the water during April and May; Paederus fuscipes is a toxic
organism with pederin in its body（Kurosa, 1958; Borroni et al., 1991）. The similarity in coloration
as well as the shared size and habitat suggests that S. flavipes nymphs may mimic P. fuscipes. The
effect of mimicry on predators remains to be determined.

Acknowledgements
I would like to thank Shinji Sugiura for the discussion on the ecology of Sirthenea flavipes.

References
Akino, K., M. Sasaki and D. Okamoto（1956）Studies on mole-crickets attacking rice seeds directly
sown without transplanting between rows of wheat and barley. Chugoku Nogyou Shikenjyo
Houkoku, Bunsatsu A, Futu-nouji-kankei,（4）: 91 ― 110.（in Japanese with English abstract）
Borroni, G., V. Brazzelli, R. Rosso and M. Pavan（1991）Paederus fuscipes dermatitis. A
histopathological study. The American Journal of Dermatopathology, 13（5）: 467 ― 474.
Hudson, W. G.（1987）Ontogeny of prey selection in Sirthenea carinata: generalist juveniles become
specialist adults. Entomophaga, 32（4）: 399 ― 406.
Ishikawa, T., M. Takai and T. Yasunaga（2012）A field guide to Japanese bugs III. Zenkoku Noson
Kyoiku Kyokai, Tokyo.（in Japanese with English title）
Kurosa, K.（1958）Studies on the life history of Pederus fuscipes Curtis（Staphylinidae）（Studies on
poisonous beetle, III）. Japanese Journal of Sanitary Zoology, 9（4）: 245 ― 276.（in Japanese withEnglish abstract）
Yasunaga, T., M. Takai, I. Yamashita, M. Kawamura and T. Kawasawa（1993）A field guide to61

Masakazu HAYASHIJapanese bugs: Terrestrial Heteropterans. Zenkoku Noson Kyoiku Kyokai, Tokyo.（in Japanesewith English title）

野外および室内観察に基づくキイロサシガメの生活史の解明
林成多

島根県出雲市で 6 月に土中から採集したキイロサシガメ Sirthenea flavipes（Stal, 1855）を飼育した結果，1 頭のメスは卵を 30 個以上産んだ．室内飼育で卵は 2 週間程度で孵化した．7 月上旬に孵化した幼虫は 8 月中旬に終齢になった．キイロサシガメの幼虫はケラ Gryllotalpa orientalis
Burmeister, 1839 の幼虫を捕食した．齢数は 5 である．幼虫の齢は，サイズや翅芽の形，頭部の色彩で判別することができる．野外でも 7 月より若齢幼虫が出現したが，同時にケラの若齢幼虫が観察された．8 月下旬には，腹部が顕著に膨らんだ，老熟した 5 齢幼虫が野外で確認され，9月上旬に羽化が飼育下で確認された．幼虫では越冬せず，成虫のみで越冬していると推定される．
また，本種の幼虫は，体色が似ているだけでなく，2 ― 3 齢幼虫の体サイズもアオバアリガタハネ
カクシ Paederus fuscipes Curtis, 1826 に近く，同じ環境に生息することから，有毒生物であるこのハネカクシに擬態している可能性を指摘した．
キーワード：ビオトープ，半翅目，カメムシ目，サシガメ科，湿地，アオバアリガタハネカクシ

62