Recent Advances in Insect Embryology in Japan
Edited by H. Ando and K. Miya.
ISEBU Co. Ltd., Tsukuba 1985

239

Percent Developmental Time (% DT); an Index to
Indicate Numerically Sequence of Events
Occurred in the Course of Embryogenesis
in Insects

Hajime MORI

Synopsis
Percent developmental time (% DT) is an index which indicates numerically chronological sequence ofthe events occurring during the embryogenesis in insects. The time of occurrence of an embryonic eventin reference to developmental time may be expressed in percentage of the entire developmental time,with 0 % DT at oviposition.
By selecting events for % DT calculation, the author laid emphasis on; 1, morphogenetic movement
which seems to have special meaning on organogenesis, and 2. histological differentiation occurring within the embryos. Information necessary to calculate % DT of these events was collected from the works on
the embryogenesis of several insects belonging to hemipteroid and orthopteroid orders.
The author suggests that introduction of % DT is rewarding and helpful to reveal basic developmental
program which may likely govern insect embryogenesis, and may further shed more lights on the understanding of evolution of Insecta.

Introduction
Previously published descriptive works on insect embryology used to give chronologicalinformation on the events with the style that such and such an event may be observed tooccur some hours or days after oviposition (Anderson, 1972a, 1972b). On the time-table ofembryogenesis appeared in these articles several embryonic events were seen to occur "collectively" at a certain date after oviposition; detailed chronological sequence of the eventsin due course of the embryogenesis however may not usually be known. Furthermore, cultural "Conditions under which these eggs were observed were different from material to material and from author to author. As the situation being so, comparison of these information
may not so easily be done especially when one wishes to reveal basic developmental program
which may likely govern the embryogenesis of taxonomically different materials.

240

Mori, H.

The author therefore intends to introduce an index, percent developmental time (% DT)
to indicate numerically chronological sequence of the events occurring during the course ofembryogenesis. The time of occurrence of certain embryonic event in reference to developmental time may be expressed in percentage of the entire developmental time, with 0 % at
oviposition and 100 % at hatching.
An attempt to indicate numerically the occurrence of the embryonic events in relation todevelopmental time was however first appeared as early as in 1960 (Striebel,); his idea isexactly the same as that of the present author, who proposed the term, % DT. Striebel(1960) compared the embryogenesis of two species of Isoptera (Kalotermes flavicollis andZootermopsis nevadensis), two species of Orthoptera (Tachycines asynamorous and Gryllus
domesticus), one species of Odonata (Platycnemic pennipes), and indicated that chronological sequence of the events was identical among the materials compared, but % DT of each
event was sometimes considerably different from material to material.
The author believes that Striebel's idea is very important and promising as an approach toreveal basic developmental program. However, insect embryologists seem to have paid very
little attention to his work, and further attempt to consider the insect embryogenesis fromthis viewpoint was not known so far (see also Bentley et al. 1979). In this article the author
will attempt to indicate quantitatively diversities observed in the embryogenesis of insects
belonging to hemipteroid as well as orthopteroid orders. Information necessary to calculate% DT of several embryonic events was collected from the works by previous authors.
Striebel (1960) seemed to have put emphasis on gross morphological features of the embryos on selecting the embryonic events to calculate % DT, whereas the present author laid
more emphasis on; 1. morphogenetic movements which seem to have special meaning onorganogenesis, and 2. histological differentiation occurring within the embryos.
Embryonic events chosen in this article are;
1. inner layer formation; this indicates interblastemic differentiation in the germ band(GB) resulting in mesoderm formation.
2. stomodaeum formation; this morphogenetic movement occurred in the ectoderm
indicates the beginning of alimentary canal formation from the anterior end of theembryo in most insects.
3. proctodaeum formation; this indicates the beginning of the alimentary canal formation from the posterior end of the embryo.
4. neuroblast differentiation; this indicates intrablastemic differentiation within theectoderm, which may lead to the ventral nerve cord formation.
S. katatrepsis; this indicates rupture of the extraembryonic membranes and subsequent movement of the embryos over the posterior egg pole.
6. yolk engulfment; this indicates zipper-like fusion of the external epithelium of theembryos has completed.

Results and Discussion
1.

Percent developmental time of embryonic events observed in several hemipteroid insects

Information of the embryogenesis of four species belonging to four families of Heteroptera, one species of Homoptera and one species of Psocoptera was analyzed. Besides the em-Percent developmental time of insect embryo

241

bryonic events noted in introduction, invagination of the blastoderm and cardioblast differentiation were employed; the former event indicates the beginning of GB formation, andthe latter event shows intrablastemic differentiation occurred within the mesoderm whichleads to the dorsal vessel formation. It should be here noted that % DT may fluctuateconsiderably for various reasons, the most significant one of which is interval of observation.
Long observation interval such as 24 hr may not give us accurate information necessary to
calculate correct % DT when duration time of the embryogenesis was from five to sevendays; for this reason observation on Pyrrhocoris (Seidel, 1924) was not included in thisarticle.
As seen in Table 1, chronological sequence of the embryonic events occurring in thesehemipteroid insects is identical. Inner layer formation was seen to occur following invagination, and neuroblast differentiation prior to stomodaeum formation; this was also true incase of Gerris. In Gerris the neuroblasts were seen to appear in the ectoderm within six hr(25°C) after embryonic rotation around the longitudinal egg axis which occurred at about
48 hr (20°C) after oviposition, and stomodaeum formation occurred subsequently, between
six and nine hr (25°C) after embryonic rotation. This indicates that % DT of neuroblast
differentiation in Gerris is not smaller than 17.4. The same sequence of neuroblast defferentiation and stomodaeum formation may also be seen from observation on Oncopeitus (Butt,
1949). However, in Pyrilla (Sander, 1956) no conclusive evidence was obtained, whichindicates that the neuroblast differentiation occurred prior to stomodaeum formation. Ectodermal cells in Pyrilla that are destined to become the neuroblasts were already observedin 35-hr embryo, but the cell division which actually produced the neuroblasts seemed tohave occurred by 41 hr, whereas the stomodaeum was already formed in 41-hr embryo.
Although % DT diversity of neuroblast djfferntiation and that of stomodaeum formation
were insignificantly small throughout these hemipteroid insect embryos, invagination of GB
in Gerris was seen to occur earlier than that in other hemipteroid insects; this means thatcomparatively long time is necessary in Gerris embryos from invagination to neuroblast
differentiation. This may be as due to either the rate in the developmental speed in Gerris
embryos has suddenly reduced after inner layer formation, or a short qUiescent period ofdevelopment has included in the course of the embryogenesis.
The author is now inclined to support this second interpretation for follwing reason.
Embryonic rotation around the longitudinal egg axis in Gerris which occurred almost
immediately before neuroblast differentiation is the type of morphogenetic movement
which is not known to occur in other hemipteroid insect embryos chosen in this article. Thisembryonic movement in Gerris began at about 40 hr (20°C) after oviposition (% DT =14.5) and finished within next six to eight hr (% DT = 17.4); no significant histologicaldifferentiation was seen to occur during this embryonic movement (Mori,1969). Thus it isvery likely that this embryonic rotation is the quiescent period specifically included in thedevelopmental program of Gerris embryogenesis. A posture taken by the embryo at the timeof hatching might have necessitated this morphogenetic movement, but true reason of inclusion of this step in the course of Gems embryogenesis is not yet known.
The cardioblast differentiation which may lead to dorsal vessel formation was an embryonic event which occurred considerably later than the neuroblast differentiation. In Gems
the cardioblasts were seen to differentiate from the somatic mesoderm immediately before
katatrepsis; this was also true in the case of Hygia (Unpublished observation). Although %DT of the cardioblast differentiation obtained in these embryos was not so close with eachMori, H.

242

Table l. Percent developmental time of embryonic events observed in several hemipteroid
insects.
* % DT may not be calculated as the time of hatching was not noted.
** % DT is underestimated (see also text).
Orderllcmiptera-I f eteroptera

FamilyLygacidae

Reduviidae

Coreidae(;enlls/spccics()nc:opeltI/J

Rhocillills[a!iciotus

prolixlls

Butt( I 949)

A\lthor(year)

Jlollloplera

Psocoptera

Gerridae

Fulgoridae

Liposcelidae

lIygioopaca(;erri,~

PyrillaLiposcelis

pallial/m

perpllsilladil'ergell,fMell.llhy( 1936)

Mori(Unpub.)Mori( 1969)

Sander( 1956)

Goss( I 953)

25

~o

:ll

31

8.9

12.7

14.3

Temp. ('C)

20

21

Invllginatioll

(35)'

11.5

Inner layer

(54)

12.4

13.2

II.J

16.2

16.1

(59)

17.2

17.5

+ 17 .4"+ 18.9"18.5

Stomodaeum
formation

(65)

20.7

21.3

+17.4

+21.6

+21.4"

Prododaeulll
formation

(65)

24.1

21.3

+17.4

+21.6

+21.4

44.8

42.3

34.8

formation

Neuroblast
differentiation

Cardioblast
differentiation
Katatrcpsis

(100)

39.7

46.1

39.9

42.2

53.0

Yolk ~ngulfl11ellt

(110)

51. 7

53.8

47.7

530

59.5

Table 2. Percent developmental time of embryonic events observed in severalorthopteroid
insects.
Order

Orthoptera

FamilyAcrididae

Isoptera
Rhaphidophoridae

Kalotermitidae

Termopsidae

Tachydnes
osy"omorrlS

Zootermopsis

Author(ycar)

LoclI.sta
migrator;oRoonwal( I 937)

J\aiolermes
j7ol'icollis

Ibrahill1(1957)

Strichel( 1%0)

Striehcl( 19(0)Temp.(n

33

26

26

26

Inner layer

13.5

IJ.O

12.9

12.5

16.7

24.0

31.5

35.7

Genus/species

lIe1'adensis

formation

Stomodaeulll
formation

Proctodaeu III

44.4

18.9

formation

Neuroblast
differentiation

20.5

29.7

40.7

Katatrepsis

46.2

59.5

57.5

56.0

Yolk engulfment

69.3

70.3

74.0

73.5

Percent developmental time of insect embryo

243other, the time from the occurrence of cardioblast differentiation to katatrepsis was almost
the same for both species. In Rhodnius the cardioblasts were said to differentiate after
katatrepsis had began (Mellanby, 1936), but this information should be reconfirmed. In
passing, the cardioblasts in Locusta (Orthoptera : Acrididae) were also seen to differentiate
before katatrepsis (Roonwal, 1937)'!'All of these hemipteroid insect embryos undergo katatrepsis; only those embryos which
underwent katatrepsis successfully are allowed to have larval structure with normal threedimensional sequence. Experimental suppression of katatrepsis has resulted in formation of
the everted embryos, in which three-dimensional sequence among the embryonic tissues wasreversed (Ando, 1955; Mori, 1975). Percent developmental time of this important morphogenetic movement in these hemipteroid insect embryos distributed from 39.7 to 53.6
and that of yolk engulfment from 47.7 to 59.5. This seems to indicate that the rate in thedevelopmental speed of the embryos has become different from material to material after
the stomodaeum formation had occurred, but entire duration time from katatrepsis to yolk
engulfment was approximately the same throughout these hemipteroid insect embryos.
This study of % OT of several embryonic events occurring in these hemipteroid insects
may suggest that they are embryologically rather uniform group. That is, they share basic
developmental program and only minor modifications of the program could be observed,
which are species-specific; the different rate in the developmental speed from one embyonic event to another may also reflect embryological species-specificity.
* %OT = 35.8
2.

Percent developmental time of embryonic events observed in several orthopteroid
insects

Information on the embryogenesis of two species belonging to two families of Orthoptera
and two species belonging to two families of Isoptera was analyzed (Table 2).
Comparison of % OT information of the two termite species has indicated that % OT
diversity was insignificantly small (see also Striebel, 1960). However, % OT diversity of theembryonic events observed in two species of Orthoptera was significantly large. Available
information was very fragmentary, but the cause of this diversification between these two
species might be as largely due to different rate in the developmental speed of these insect
embryos.
In these orthopteroid insect embryos stomodaeum formation was seen to occur before
neuroblast differentiation. Occurrence of the proctodaeum was seen to follow the stomodaeum formation, but it was difficult to determine whether the proctodaeum formation had
started before the neuroblast differentiation or not; this should be considered as due to thedifficulty in determining the exact time of occurrence of proctodaeum formation.
Katatrepsis in these orthopteroid insect embryos was seen to occur later than that in thehemipteroid insect embryos (Table 1). Although the identical style of embryonic movement was observed in both groups of these insects at the time of katatrepsis, the time necessary from katatrepsis to yolk engulfment in orthopteroid insect embryos was much longer
than that in hemipteroid insect embryos. The author has obtained an impression thatconsiderably long time necessary to finish the zipper-like fusion of the external epithelium
observed in these orthopteroid insect embryos brought about this prolongation of yolk

Mori, H.

244

engulfment procedure.
Difference of % DT of the embryonic events obtained from these orthopteroid insects
may suggest that the insects included in the orthopteroid group are embryo logically morediverse than those belonging to the hemipteroid group.

3.

Discussion

The striking difference found between the results obtained by the present author andthose by previous author (Strievel, 1960) is that chronological sequence of stomodaeum
formation and neuroblast differentiation in the course of embryogenesis is reversed between
the insects belonging to different orders. In the most hemipteroid insects observed, neuroblast differentiation occurred prior to stomodaeum formation, whereas stomodaeum formation in orthopteroid insects occurred earlier than neuroblast differentiation. This mightsuggest that the style of GB formation is influential in the chronological sequence of theevents which occur subsequently, as hemipteroid insect embryos employ immersion type
GB formation, whereas orthopteroid insect embryos employ superificeal type GB formation.
However, what the author intends to emphasize here is that % DT could be employed
as one of the most useful information in representing diversifications of the embryogenesis
among different insect orders, and at the same time, may help us to understand basic developmental program most likely govern insect embryogenesis.
It is reasonable to assume that insect embryogenesis may proceed under the direction ofbasic developmental program, in which all developmental events (steps) are arranged sequentially. However, when we observe individual material we are only allowed to know various
modifications of this program, each of which is specific for each insect species. These modifications on basic developmental program must have occurred in the course of evolution of
Insecta either as a change in the chronological sequence of the events, as seen in case of
neuroblast differentiation and stomodaeum formation described in this text, or change inthe pattern of cytodifferentiation, i. e., morphological diversification. Appearance as well
as disappearance of events occurred in the developmental program, as seen in the case of embryonic rotation specifically included in the developmental program of one group of Hemiptera, may also be considered as another cause of modifications. It is very likely that one
type of modification occurs in close association with another.
Up to present insect embryologists tended to evaluate results of morphological observation as the only available and worthy information to interprete embryogenesis in insects.
This is especially true when previous authors considered phylogeny of Insecta. However, it
is very difficult to decide which one of the morphological features is basically important
and which one has occurred as a result of diversification. From this viewpoint the author
considers that accumulation of quantitative information such as % DT is very rewarding
and helpful to reveal basic developmental program of insect embryogenesis, and consequently contribute to the understanding of evolution of Insecta.
Finally, it should be pointed out that % DT may only indicate sequence of occurrence ofthe events in due course of the embryogenesis; this index however is useless to indicate theheritage of individual cells consisting each embryonic organs. Cell lineage which has beenused to indicate developmental history of cell differentiation, on the other hand, failed toinclude chronological information; this is the critical shortcoming in considering embryonicPercent developmental time of insect embryo

245

developmental program. The author therefore suggests that cell lineage and % DT should be
integrated in the near future to describe embryogenesis in insects.

References
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& Waddington, C. H. (eds.) Developmental Systems - Insects vol. 1, 95-163, Academic
Press, London & New York.
_ _ _ _ _ _ . 1972b The development of holometabolous insects. In : Counce, S. J.
& Waddington, C. H. (eds.) Developmental Systems - Insects vol. 1, 165-242, Academic Press, London & New York.
Ando, H. 1955 Everted embryos of dragonflies produced by ligation. Sci. Rep. Tokyo
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1953
The advanced embryology of the book louse, Liposcelis divergens
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Normal embryogenesis of the waterstrider, Gerris paludum insularis
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1956 The early embryology of Pyrilla perpusilla Walker (Homoptera), inSander, K.
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Author's address:

Dr. H. Mori,
Department of Natural History, Facultyof Science, Tokyo MetropOlitan University, Setagaya-ku, Fukazawa, Tokyo 158,
Japan.