Archives of Insect Biochemistryand Physiology 19:119-132 (1992)
The Oviduct Musculature of the Stable Fly
Stomoxys calcitrans: Properties of Its
Spontaneous Motility and Neural Regulation
Benjamin J. Cook
U.S.Department of Agriculture, Agricultural Research Service, Food Animal Protection Research
Laboratory, College Station, Texas
The muscles of the stable fly oviduct were striated and in the common oviduct both circular and longitudinal fiber layers were present. Although two
muscle layers were evident in the lateral oviducts, both consisted of an irregular lattice of longitudinal fibers with extensive branching. This branching
between fibers seemed to provide a pathway for the spread of excitation
because semi-isolated preparations of the oviduct severed from their connections to the central nervous system continued to contract in a spontaneous and rhythmic manner. Compression or shortening of the longitudinal
muscles of the oviduct was the predominant mode of activity, Both phasic
contractions (1-2 s in duration) and tonic events (15-30 s in duration) were
observed. Innervation of the oviduct arises from the four major branches of
the median abdominal nerve trunk that enter the ovipositor. Five to seven
peripheral nerve cells were found along these four branches. Electrical stimulation of the severed median abdominal nerve trunk caused the oviduct to
respond to a single pulse. However, these individual responses approached
a fusion at 6 pulses per second (pps). Two of five preparations showed an
inhibition of spontaneous phasic concentrations and a noticeable drop in baseline tonus during and after a 10-20 s interval of stimulation at a frequency of
0.5-2 pps. Endogenous electrical activity was recorded from branch nerves in
the ovipositor after connections to the central nervous system were cut. Potentials of two distinct durations were evident in this recorded activity. The
significance of this and other findings are discussed.
Key words: Diptera, myogenic, neurogenic, visceral muscle, peripheral nerve cells
INTRODUCTION
The oviduct has long been recognized as the principal organ facilitating egg
deposition in insects but the physiological details of the process and the simple facts of muscle movement in that organ are fragmentary to nonexistent for
Received June27,1991; accepted October 29,1991.
Acknowledgments: The author would like to thank Tara Petersenfor excellent technical assistance
and for figure presentation and drawings.
Address reprint requests to Benjamin J. Cook, US. Department of Agriculture, Agricultural
Research Service, Food Animal Protection Research Laboratory, Route 5, Box 810, College Station, TX 77845.
0 1992 Wiley-Liss, InC.
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Cook
most members of the class. Some of the more comprehensive studies to date
have been done on orthopteroid insects. Thomas [l]has shown that the central nervous system of the stick-insect Curuusius morosus regulates egg progression in the common oviduct. He found that the contraction of a specialized
semicircular sheath of muscles in the common oviduct causes a constriction
in the lumen of the duct that prevents egg movement. Moreover, the reflex
activation of this muscular sheath appears to be triggered by sensory impulses
from hairs in the wall of the egg chamber. In the locust, Locustu migratoria, it
has been determined that electrical stimulation of the nerves leading to the
oviduct causes contractions of the common and laver lateral oviducts [2]. These
contractions in turn propel ovulated eggs back towards the ovaries. Moreover,
chronically implanted electrodes on these nerves reveal that ,electricalactivity
is lav during the process of oviposition but high when egg laying does not
occur [3]. Thus, it was concluded that one function of the aviducal nerves was
to regulate egg deposition by evoking contractions in the lower oviduct. Such
events provide a physiological basis for part of the adaptive ovipositional behavior of this insect. The process of oviposition in the locust, however, appears to
be complicated by the fact that there is evidence that hemolymph from
ovipositing females contains a hormonal factor which can stimulate spontaneous contractions of the oviduct [4]. Moreover, the ducts that contain mature
eggs respond to homogenates of the corpus cardiacum but oviducts that are
devoid of eggs do not. In summary, it appears that neuroendocrine factors
can initiate miposition but the neural circuit that suppresses egg transport
down the oviduct must be switched off for the process to proceed.
Like the examples just described, the oviducts of many insects are composed
of striated muscles that are subject to both neural and hormonal regulation,
and in many ways such muscles parallel the functional properties of the smooth
muscles of vertebrates. The present study represents an attempt to establish
some fundamental facts about the oviduct of the stable fly that might be useful in understanding the complex process of oviposition in a dipterous insect.
The focus has been to: 1)describe some of the structural relationships between
the nerves and muscles of the oviduct and associated organs, 2) investigate
the nature of spontaneous contractile events in the oviduct, and 3) determine
the principal properties of neurally evoked events.
MATERIALS AND METHODS
Adult female stable flies (Stomoxys cutcitruns) were obtained from a laboratoryreared colony at the USDA, ARS, Food Animal Protection Research Laboratory, College Station, Texas. In this colony, both male and female flies were
held together in screen cages at 27°C and 50% humidity. The flies were fed
daily by placing cotton pads soaked with citrated bovine blood on top of the
screen cages. At appropriate times after adult emergence (6, 7, and 8 days),
female flies were removed from cages in the colony. The composition of the
saline used for dissection and perfusion was (in mM) NaCllO5, KN035, CaC12
5, MgS04-7H203, L-histidine 10, and glucose 28. The pH was adjusted to
6.8 with sodium hydroxide. Composition of the saline solution reported here
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was developed from a recent analysis of cations and anions found in adult
stable fly hemolymph [5].
Preparation of Tissue for Microscopic Examination
The muscle networks and nerves of the oviduct were stained by placing a
drop of 0.25% methylene blue (Fisher Chemical Co., Fair Lawn, NJ) on ventrally exposed ovaries in the abdomen of the fly for 15-30 min. The staining
solution was prepared daily by making a 1:l dilution of a stock solution of
methylene blue (0.5% distilled water) with stable fly saline. Once the oviducts
were sufficiently stained, the preparations were iinsed several times with a
solution of ammonium molybdate (0.8%) to retard the loss of stain from muscle cells and nerves. The cellular networks were examined by Nomarski interference and ordinary light microscopy.
Preparation of Oviduct and Peripheral Nerves for Recording
Female flies were immobilized by chilling and the head, wings, and legs
were removed. The bodies that remained were placed in a small wax-filled
Petri dish, and pinned through the ventral surface of the thorax. The minuten
pin was thrust through the thorax close to the articulation of the wing to avoid
striking the thoracic ganglion. At this point, the ovipositor was drawn out and
pinned to the wax preparation dish with a minuten pin. The central portion
of the integument on the ventral abdomen was then cut out. The body cavity
was flooded with stable fly saline and the integument of the abdominal side
walls was pinned with a minuten on each side. This procedure was followed
by cutting the integument of the ovipositor along the midline to give more exposure to the terminal end of the common oviduct and the branch nerves that
run parallel to it. This type of preparation was found quite suitable for extracellular nerve recordings explained later.
When spontaneous muscular activity of the oviduct was to be recorded, the
digestive tract was carefully removed by severing the hindgut from its close
apposition to the oviduct near the spermatheca. The entire oviduct was then
exposed by drawing it in a posterior direction while cutting the tracheal attachments. After the ovipositor was pinned in wax as described and the integument of that structure opened along the midline, the oviduct was almost
completely exposed as required for recording (see Fig. 3A). A monofilament
of nylon was tied below the midline of the ovaries and the other end of the
filament was attached to the beam of the load cell (Kulite BG 10, Kulite Semiconductor Products, Leonia, NJ). The details on the use and calibration of
this miniature force transducer system have been described elsewhere [ 6 ] .Once
the oviduct preparations were arranged in the 100 pl bath of saline, they
remained active for as long as 6 h with only occasional changes of saline.
Video recordings of oviduct motility were obtained by focusing a Panasonic
video camera through a dissecting microscope on the dorsal surface of the
suspended tissue. Timed sequences of movement were taped on a video cassette recorder for irnage-by-image analysis.
Innervated oviducts were suspended in a 100 pl saline bath as described for
preparations used in recording spontaneous muscle activity. A suction elec-
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Fig. 1. Drawing of the major anatomical features of the stable fly oviduct shown in dorsal
aspect.
trode (tip diameter 50 Fm) was placed in close proximity to the semi-isolated
oviduct and the severed end of the median abdominal nerve was drawn into
it. The electrode was connected to a Grass 588 stimulator (Grass Instruments,
Quincy, MA) through an isolation unit, and the circuit was completed by
placing an indifferent silver chloride electrode in the saline bath.
Spontaneous nerve potentials were recorded from in situ preparations of
the oviduct (described above) by drawing small sections of the branch nerves
Stable Fly Oviduct
123
that run along the ovipositor into saline filled suction electrodes. The electrode tips were made of capillary glass with a tip diameter of 40-60 pm.
The extracellular recorded signals were amplified by a Grass P511 through
a high-impedance probe and recorded on a Gould 1602 digital storage oscilloscope (Gould Recording Systems Division, Cleveland, OH), Signal amplitudes and durations were measured by time and voltage cursors.
RESULTS
Muscle Networks of the Oviduct
As in most insects, the oviduct of the stable fly is divided into two distinct
regions (Fig. 1):1) the lateral oviducts which consist of a pair of translucent
muscular tubes that extend from the calyx near each ovary to their connection
with the common oviduct; and 2) the common oviduct which extends in a
posterior direction from its junction with the lateral oviducts to the anterior
vagina just beneath the spermathecal ducts.
Once the lateral oviducts are stretched to reveal the fine points of their structure, it is evident that each tube consists of an inner muscular sheath enveloped by an outer one. The outer sheath of fibers not only covers the individual
tubes of the lateral oviducts, but also provides a broad central v-shaped connection between them (Fig. 2A,B). Although this central region has both a
dorsal and ventral surface of fibers, mature eggs that pass down either tube of
the lateral oviducts never enter this region.
All the muscles of the oviduct are striated yet a distinctive pattern is evident in each region. In the common oviduct, both circular and longitudinal
fibers are present and they are arranged in separate layers (Fig. 2C). The c i r
cular fibers appear largely on the surface in tightly arrayed oblique profiles
that crisscross one another while the longitudinal fibers are more deeply
situated.
The muscle layers of the lateral oviduct are quite thin compared to those
found in the common oviduct and the orientation of the fibers in each layer
can best be described as an irregular lattice of longitudinal fibers with substantial branching. The muscles that comprise the outer sheath of the lateral
oviduct either consist of a continuation of the fiber network that covers the
surface of each ovary [7] or muscle bands that originate from the attachments
of accessory glands to the wall of the lateral oviduct. When mature eggs are
present in the ovary, the outer muscular sheath of that organ can often be
seen in profile stretching across the calyx and lateral oviduct (Fig. 2D) and
careful focusing of the microscope on the upper surface of the lateral oviduct
itself can confirm the presence of this network (Fig. 2E). An inner sheath of
longitudinal fibers that is continuous with the egg tubes of each ovary guides
the mature eggs through the lateral oviducts. A field of these fibers of the
inner sheath is shown in Figure 2F.
Spontaneous Activity and Myographic Recordings
Spontaneous and rhythmic contractions were observed in the majority of
the semi-isolated preparations of the stable fly oviduct after connections to
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Fig. 2. Visceral muscle networks of the oviduct as revealed by Nomarski interference microscopy. A: Ventral aspect of the muscular sheaths in common (CO) and lateral oviduct (LO). B:
Close-up of muscle fiber branching (arrow) in the sheath between the lateral oviducts showing striations and the four major nerve branches ( N I , 2, 3, and 4) that arise from the median
abdominal nerve trunk. C: Photomicrograph of circular muscle fibers on the surface of the
common oviduct with more deeply set longitudinal fibers (arrow). D: Profile of the muscle
network stretching from the surface of a mature ovary (Ov) across the calyx (Clx) to the outer
E: The same muscle network shown in D on the outer sursurface of the lateral oviduct (LO).
face of the calyx (Clx) and lateraloviduct of another preparation. F: More deeply situated branching longitudinal muscle fibers (arrows) in the calyx (Clx) and lateral oviduct (LO).
the central nervous system were severed. The evident branching between muscle fibers in the oviduct suggests that a pathway exists for the spread of excitation throughout the tissue, regardless of neural input. Thus most spontaneous
contractions can probably be classed as myogenic but subject to neural
regulation.
The often rapid and complex character of these observed movements could
be resolved by image-by-image analysis of taped video sequences of the activity. Compression or shortening of the Iongitudinal muscle fibers in either the
common or lateral oviducts was the predominant mode of activity. The location and sequence of this activity are indicated in Figure 3A. The compressions were particularly evident in the outer sheath of the lateral oviduct and
at the distal end of the common oviduct. Both phasic contractions (1-2 s in
duration) and slower tonic events (15-30 s in duration) were observed. Fortu-
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125
A
4 See
Fig. 3. Spontaneous motile actions of the oviduct and myographic recordings of longitudinal
muscle contraction patterns. A: Drawings show the location and sequence of compressions
(shaded area) that occurred in a 4 s video recording of oviduct activity. Calyx (Clx), common
oviduct (CO), lateral oviduct (LO); ovipositor (Op), and spermatheca (Sp). B-F: Myographic
records of five different patterns of spontaneous activity observed in 7 and 8 day flies.
nately, myographic recordings also gave an accurate record of these activity
patterns.
A profile of the range of these rhythmic patterns is shown in Figures 3B-F.
Examples of slow rhythmic changes in tonus of the common oviduct are shown
in Figures 3B and C. The small, rapid phasic contractions in Figure 38 occurred
in the calyx region of the lateral oviduct while the larger phasic contractions
in Figure 3C took place in the common oviduct. Slow changes in tonus were
also evident in the lateral oviduct and an example of this is shown in Figure
3D. In this particular instance, tonus increased and was sustained for almost
1 min in the region of the calyx. The small phasic contractions that occurred
during the elevation of tonus originated in the ovarian sheath while the much
larger spike-like contractions occurred in the common oviduct. Occasional
sequences of irregular contraction amplitude and frequency were observed in
the oviduct (Fig. 3E) and brief periodic relaxations of base-line tension were
also detected (Fig. 3F).
Innervation Patterns and Neurally-Evoked Responses
The abdomen of the stable fly receives its innervation from a single nerve
trunk, the median abdominal nerve. This nerve arises from the thoracico-
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A
Fig.4. Principalstructural featuresof the branch nerves, peripheral nervecells, and innervation
networks associated with the oviduct of the stable fly. A: Dorsal lateral aspect of the oviduct
and ovaries showing the position of ganglia along branch nerves and the major fields of muscle innervation on the oviduct (number circles). 6: A peripheral cluster of two cells along
a branch nerve near the common oviduct. C: Close-up of a pair of nerve cells along another
branch nerve in the ovipositor. D: Innervation network on the surface of the muscle sheath
between the lateral oviducts. E: Small nerve branches directed towards the surface of the common oviduct, Note small nerve cell (arrow). F: Close-up of small branch nerves leading to the
junction between the common (CO)and lateral oviducts. Note the close apposition of tracheoles
to nerves (arrows).
abdominal ganglion and gives off paired branches to each segment in its posterior course. As this nerve trunk approaches the posterior end of the 4th
abdominal segment, it gives rise to four major branches that enter into the
ovipositor. These four branches are shown crossing the mid-region of the lateral oviduct in Figure 2B. Two of the branches pass over the upper surface of
the muscular sheath while the other branches go beneath. As these four
branches descend into the ovipositor, they give off numerous smaller branches
that innervate the hindgut, oviduct, spermatheca, vagina, and muscles along
the wall of the ovipositor. The drawing in Figure 4A depicts the major features
of this innervation network. Several peripheral nerve cells are present in this
network and the microscopic features of some of them are shown in Figures
4B, C, and E. These nerve cells were never directly attached to either the sur-
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Fig. 5. Mechanical response of the oviduct to neural stimulation. A Changes in oviduct
response to increases in stimulation frequency. From left to right, the frequencies are 1, 2, 3,
4, 5, and 6 pps in 1 s trains. B,: Oviduct contractile changes in another preparation at higher
stimulation frequencies. From left to right 5,10,15,20, and 30 pps in 1 strains. B2: Response
of the same oviduct to a 20 s interval of continuous stimulation at 0.5 pps (bar). 6,: Response
of the same preparation to a20 s interval of continuous stimulation at 2 pps (bar). C: Response
of another oviduct to 15 s of continuous stimulation at 1 pps (bar).
face of the hindgut or the oviduct but remained largely associated with the nerve
branches themselves and in some preparations numbered as many as seven.
A high density of nerve endings is found in three major areas of the oviduct: 1)around the distal end of the common oviduct; 2) along the proximal
end of the common oviduct and around its junction with the lateral oviduct;
and 3) in the region of the calyces (Fig. 4A). The microscopic features of several of these areas are shown in Figures 4D-F. The nerves approaching the
oviduct and their endings on muscle fibers are frequently accompanied by
tracheae and tracheoles.
When the severed median abdominal nerve was stimulated by a single electrical pulse of a 0.4 ms duration, the longitudinal muscles of the oviduct
responded with a simple monophasic contraction that lasted approximately 1
s (Fig. 5A). The delay between the stimulus and the mechanical response of
the muscle was generally less than 125 ms. If the frequency of the stimulating
pulse was increased as shown in Figures 5A and B1 the magnitude of the oviduct response increased. Initially at 2 pulses per second (pps), the individual
responses could be clearly distinguished but as the stimulus frequency
approached 6 pps/ a fusion of contractile events began to occur and the amplitude had doubled. In some preparations, the amplitude of contraction continued to increase until a maximum was reached at a frequency of 30 pps (Fig.
5B1). In two of the five innervated preparations of the oviduct that were stud-
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ied, an inhibition of spontaneous phasic contractions and a noticeable drop
in base-line tonus occurred during and after a 10-20 s interval of neural stimulation at a frequency of 0.5-2 pps (Fig. 5B2,B3,C).Stimulation of the oviduct
shown in Figure 5B2-3at low rates for 20 s caused an initial elevation in tonus
followed by a slight (at 0.5 pps) and a large (at 2 pps) drop in tonus that lasted
almost 50 s. Both frequencies of stimulation, however, suppressed spontaneous contractions of the lateral oviducts. Another preparation of the stable fly
oviduct (Fig. 5C) showed a quite uniform phasic response to neural stimulation
(at 1pps), followed by a 15s inhibition of activity and a moderate drop in tonus.
1
16auv
I
I
Fig. 6 . Endogenous electrical activity from the thoracico-abdominal ganglion and peripheral
nerve cells along branch nerves in the ovipositor. A: Activity on the median abdominal nerve
trunk with the thoracico-abdominal gangiion intact (top trace). Note burst of higher amplitude potentials. Faster scan at the same recording site (middle trace). Electrode in saline solution just above preparation (bottom trace), B: Recording from branch nerve near the common
oviduct with connections to central nervous system intact (upper trace). Same recording site 2
min after the abdominal trunk nerve was severed (lower trace). C: Endogenous activity along a
branch nerve on the right side of the common oviduct after connections to the central nervous system were severed (upper trace). Faster scan at the same recording site showing potentials of different duration times (lower trace). D: Spontaneous activity of peripheral nerve cells
as recorded on another branch nerve on the left side of the common oviduct after connections
to the central nervous system had been severed (upper trace). Faster scan at the same recording site again showing the different duration times of potentials (lower trace).
Stable Fly Oviduct
129
Spontaneous Activity of Peripheral Nerves
Spontaneous neural activity on the intact median abdominal nerve trunk
generally had a biphasic and arrhythmic character (Fig. 6A). However, in a
number of preparations of female stable flies recurrent bursts of potentials
with a higher amplitude were observed. These bursts of elevated activity lasted
from 2-4 s and an example of them is shown in the top trace in Figure 6A. The
bottom trace in the same figure shows the level of electrical activity when the
recording electrode had been placed in the saline solution just above the
preparation.
In an effort to demonstrate endogenous electrical activity in the peripheral
nerve cells described above, a smalI suction electrode was placed on one of
the four major branch nerves leading into the ovipositor. A recording of spontaneous activity along one of these branch nerves, near the common oviduct,
is shown in the top trace of Figure 6B. The activity in this trace reflects a considerable input from the large thoracico-abdominalganglion because once the
median abdominal nerve trunk has been severed, all the signals in excess of
50 pV disappeared and the level of activity at the same recording site showed
even a reduction in the frequency of potentials 50 pV or less (Fig. 6B, bottom
trace).
In another preparation, the recording electrode was placed on a branch nerve
along the right side of the common oviduct. In this instance, the median abdominal nerve had been severed before the recording began (Fig. 6C, top trace).
At least two types of potentials were evident at this recording site, a periodic
potential of larger amplitude and recurrent bursts of smaller and more rapid
potentials (Fig. 6C, lower trace). When the recording electrode was placed on
a branch nerve between the common oviduct and the accessory gland on the
left side of the ovipositor of another preparation isolated from the central nervous system, at least three different potential types were observed (Fig. 6D,
upper trace). Here the potential of lower amplitude had a longer duration than
the larger spikes (Fig. 6D, lower trace).
DISCUSSION
The muscular networks of the stable fly oviduct were distinctive for each
region, yet their structural features showed a close similarity to those found in
the horsefly, Tabanus sulcifrons [8] and the house fly, Musca domestica [9]. The
common oviduct, for example, has a thick outer sheath of circular muscles
with a more deeply imbedded network of lonptudinal fibers. However, confirmation on the number of muscle layers in this tissue and their exact thickness must await a more comprehensive histological study. The longitudinal
muscles of the lateral oviduct shaved extensive branching between fibers thus
forming a structural syncytium quite similar to that reported in the horsefly [8].
Certainly this structural feature provides, in part, the functional basis for
the spontaneous and rhythmic contractions observed in the muscles of the
stable fly oviduct. Such functional properties in insect visceral muscles are
usually expressed in four basic patterns or sequences of motile activity, namely
compression, peristalsis, reverse peristalsis, and segmentation [lo]. Compression or shortening of the longitudinal muscle fibers was the predominant type
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of motile activity observed in the oviduct of the stable fly. However, some of
these events were quite localized and failed to spread throughout the muscular syncytium. Myographic recordings gave an accurate profile of the temporal sequences of rhythmic contractionsand tonic changes in the oviduct because
all of the observed activity seemed to be a consequence of longitudinal muscle shortening. The range of this activity is shown in Figure 3. Examples of
peristalsis and segmentation were not detected as reported in the hindgut of
the stable fly [ll].
The general pattern for innervation of the stable fly abdomen is similar to
that described for the house fly [12] with paired segmental nerves arising from
a single median abdominal trunk. Moreover, as this trunk approaches the posterior end of the 4th abdominal segment, it gives rise to four major branches
that enter into the ovipositor (Fig. 213). As many as seven peripheral nerve
cells were found along these nerve branches in the ovipositor, but they were
never directly attached to the surface of the oviduct as reported in the pink
bollworm moth, Pectinophoru gossypiellu [13]. Such peripheral nerve cells have
been reported in the female blow fly, Phorrniu regina [14] and the male stable
fly [15]. Some of those found in the latter insect have been shown to be neurosecretory in nature. The principal features of oviduct innervation and some of
the microscopic details of the nerve cells are shown in Figure 4.
Although a single electrical pulse to nerves leading to the oviduct caused
the longitudinal muscles of that organ to contract in a simple monophasic manner, the character and magnitude of the response could be altered by increasing the stimulus frequency (Fig. 5) as reported for other insect visceral muscles
[2,16]. However, in two of five oviduct preparations, an electrical stimulation
of the severed median abdominal nerve at a frequency between 0.5-2 pps for
10-20 s caused a noticeable drop in base-line tonus and an inhibition of spontaneous phasic contraction. Such a neurally evoked response has not often
been reported to occur in insect visceral muscle and its full significance must
await further experimental work. However, it is of interest to note that recent
pharmacological experiments show that octopamine causes an inhibition of
spontaneous muscle contractions of the stable fly oviduct in the range of
10 8-10 p6 M. A large drop in base-line tonus also often accompanied this inhibitory response (Cook and Wagner, unpublished observations). The biphasic
and arrhythmic character of spontaneous neural activity on the intact median
abdominal nerve trunk of the stable fly is quite similar to that found in other
insects [3, 131. Even the pattern of recurrent bursts has been noted. In the
locust, such bursts have been linked to the inhibition of egg deposition. The
peripheral nerve cells found along nerve branches of the stable fly ovipositor
(Fig. 4) strongly suggest the presence of a functioning intrinsic neural network in that region. In an effort to demonstrate this experimentally, we severed the median abdominal nerve trunk in the anterior portion of the abdomen
and explored the branch nerves in the ovipositor near the common oviduct
for endogenous activity. Although the thoracico-abdominal ganglion provides
a considerable input to this region, endogenous activity from peripheral nerve
cells was evident (Fig. 6). In most preparations, recordings of this activity
showed a variety of potential types which suggests that the signals were being
transmitted from several different nerve cells.
Stable Fly Oviduct
131
It has been recognized for some time that the action potentials of insect
neurosecretory cells often have durations 2-20 times greater than the potentials from other neurons [17]. Some potentials recorded from the stable fly
ovipositor branch nerves seem to fall into this category because these potentials had durations of 5-6 ms while others were only 1-2 ms in duration. These
electrophysiological data together with the recent ultrastructural demonstration of such cells in the peripheral nerve cells of the male stable fly [15] offer
strong evidence of the involvement of neurosecretion in this system.
In summary, this study establishes two prospective endogenous sources of
physiologicalregulation of the reproductive system, namely the central nervous
system and a network of peripheral nerve cells in the ovipositor, Moreover, the
spontaneous and rhythmic character of the muscle contractions of the oviduct
themselves suggests the possibility of another avenue of regulation by endogenous chemicals in the hemolymph. This latter prospect seems clearly
evident from preliminary work [18], but the subject is still under investigation.
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