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Insect Exploration

by Travis Casagrande

 

What do you do when you have a fully intact dead insect and you also just happen to have an SEM? That’s a rhetorical question; we both know there’s only one answer.

If you’ve studied entomology (the study of insects), this may not be news to you, but like many of our users, I came from materials science and engineering. It’s safe to assume that we might not know a lot about insect facial features. So, I put one inside an electron microscope; my FIB column was temporarily broken so I had a bit of extra time. This insect was one of many that found its way into the house of one of our other staff members and the insects liked it so much that they decided to nest there. This required the services of pest control.

 


Alright, so what is this? I’ve already given you some major hints. Maybe you think it’s the eye of an insect?

 


There’s two; that’s a good sign for the eye theory.

 


Wait, there’s a 3rd? Uncertainty sets in. Let’s zoom out some more.

 


This is a wasp, but you only see part of the head here since it’s far too big for my maximum field of view. (In the electron microscopy world, insects are giants.) Now you can see the large “compound eyes” that you’re probably familiar with since they’re easily visible with your own eyes and likely the most recognizable insect facial feature.

 


These compound eyes only provide the wasp with low resolution vision. However, since they protrude outward in a dome and curve down the side of the wasp’s head, the shape gives them a very large 3-dimensional field of view, or more mathematically speaking a “high solid angle of detection”. They can also detect fast movement. [1]

 


Each domed hexagonal shape is an individual cornea that is transparent to visible light. Under the cornea is a cone lens, photoreceptor cells, and pigment cells to block excess light transmission to other units; all together forming a unit called an ommatidium. The compound eye consists of hundreds to thousands of ommatidia. [2]

 


Nature is so perfect with these hexagons, right?

 


Nope.

I found edge dislocations in the hexagonal arrangement of the ommatidia, and this is one of them. (Materials science, remember?) You can see by the angles of the blue lines how the hexagonal lattice distorts to accommodate the extra line of ommatidia. The hexagon angles and side lengths close to the dislocation site are also heavily distorted, with a few of them having only 5 sides. These dislocation “defects” weren’t uncommon either; there’s even a second one in the horizontal direction on the left edge of this image.

 


Anyway, back to the 3 domes in between the two compound eyes. So, then what are they?! I couldn’t leave this question burning in my mind for too long, so I tracked down the answer.

 

The answer: Also eyes!  Wait… what?
They are called dorsal “ocelli”, which is the plural of “ocellus” which means “simple eye”. Apparently, this extra triplet of eyes does not form an image, but they can detect variation in light intensity far more precisely than the compound eyes, and faster too because of their relative biological simplicity. These ocelli seem to be theorized to help with flight stability. Consider the pitch, yaw, and roll rotation axes in flight and how the difference in light intensity detected between each of the three ocelli would be enough to detect changes in all three of those rotational directions, assuming their environment has some directional lighting.

[image source: 2]

 
This is more than just a curiosity for science, because understanding the ocellar functions in insects can for example be applied to the engineering of flight stability controls in micro air vehicle drones. [4,5]

 

A few additional comments for those interested in the sample preparation of insects and some related SEM tips:

  • Usually you can expect an insect to be far dirtier than the one I have here, especially if you’ve found it dead on the floor. This wasp was even given to me covered with pesticide residue. I cleaned it thoroughly by ultrasonication in acetone, and then repeated with ethanol, and it seems to have been quite effective.
  • The most proper way to prepare an insect for SEM would be using a technique called “critical point drying” to displace the water and evaporate the fluid in a controlled way that won’t collapse the fine biological structures such as cell walls. This equipment is common in a biological-focused electron microscopy lab. With softer insects this is a critical technique, but the acetone/alcohol washing was good enough for the wasp, at least at low to medium magnification. If it was recently alive, it could be a little more “wet” inside and possibly off-gas once in the vacuum of the microscope chamber. A vacuum oven with gentle heating can be used to remove some of the remaining liquids to prevent contaminating the SEM chamber.
  • As you might expect, insects are non-conductive. I coated this wasp with argon-plasma sputter deposition of gold, and for much longer than I would for a typical flatter sample. I probably should have coated it even more thoroughly because there was still a fair bit of charging to deal with. Charging can be somewhat mitigated by low current operation and rapid frame averaging during image acquisition. The deeper areas and underside of the insect were still non-conductive, so it is best to use a coating method that can access the more difficult angles.
  • Since insects are huge, it’s helpful to use a large working distance because this provides a large depth of field so that most of the insect is at least somewhat in focus. High voltage also helps for larger depth of field, but sometimes will make charging effects worse. Applied to a low atomic number material, the high voltage beam can give an undesirable semi-transparency effect that prevents crisp imaging of surface textures, although this is solved by the metal coating.
    The large working distance also helps to avoid any collisions with the pole-piece, since getting to the various parts of the insect with multi-millimeter height differences requires large stage movements and a variety of tilt angles. Extra vigilance is needed to ensure that stage movements are done safely.

 

References:

[1] https://en.wikipedia.org/wiki/Compound_eye

[2] https://en.wikipedia.org/wiki/Ommatidium

[3] https://whyfiles.org/2015/fruit-fly-flight/index.html

[4] https://en.wikipedia.org/wiki/Simple_eye_in_invertebrates

[5] http://aeromav.free.fr/MAV07/session/plenary/SESSION%204/MAV07-PLEN%204-3%20Stange%20Berry%20Kleef/Stange.pdf

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COVID-19 Report – CCEM re-entering Phase 2

As the COVID-19 pandemic begins to improve in Ontario, and Hamilton returns from lockdown, McMaster University has eased restrictions for on-campus research operations. For the CCEM, this means that starting Tuesday Feb 16, the following activities are able to be resumed:

  • In-person user training for local (McMaster) users
  • In-person instrument access for external (non-McMaster) users, provided that they are able to work independently

The CCEM staff hopes that everyone continues to remain safe and healthy during this time. We look forward to resuming in-person training to all users when the situation allows. Thank you for your understanding.

For additional information, or if there are questions, please consult COVID-19 policies, or email the Acting Facilities Manager.

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COVID-19 Report – CCEM Scaling Back Parts of Operations

As a result of new restrictions due to the ongoing COVID-19 pandemic, the CCEM will be scaling back parts of our operations to help ensure the safety of all staff and users of the Centre.  These new restrictions will remain in effect until further notice:

  1. CCEM will no longer be offering in-person training with our assistance from CCEM staff
  2. In-person access to the CCEM will be limited to McMaster personnel only, who are working under an approved on-campus research plan

If you already have a booking for in-person use and are external to McMaster, or are scheduled to receive in-person training, your session will have to be postponed.

Detailed CCEM COVID-19 protocols can be found on the CCEM File Server (https://ccemdata.mcmaster.ca/published/ccem-covid-19/).

The CCEM also remains open to receiving jobs for analysis by our staff.

If you have any further questions regarding our limited re-opening please contact the Acting Manager.

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COVID-19 Report – CCEM entering Phase 2 of re-opening

Following McMaster’s lead, the CCEM will be moving into our Phase 2 of re-opening. What does this mean? The Centre will now be open to non-staff users that are trained to work independently and to those scheduled to receive in-person training with CCEM staff. In-person access to the CCEM requires the following:

  1. Users familiarize themselves with and follow all COVID-19 protocols in place by the CCEM, the university, and other university departments and offices – including the mandatory use of face masks for indoor, public spaces.
  2. Users book their instrument time in advance using NanoLIMS, or by arranging time with CCEM staff

Note that during this phase we are limiting instrument room occupancy to a maximum of 1 person (with an exception of 2 people for training by CCEM staff).

Detailed CCEM COVID-19 protocols can be found on the CCEM File Server (https://ccemdata.mcmaster.ca/published/ccem-covid-19/).

The CCEM also remains open to receiving jobs for analysis by our staff.

If you have any further questions regarding our limited re-opening please contact the Acting Manager.

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COVID-19 Report – CCEM will soon begin a limited re-opening

The CCEM will soon begin a limited re-opening, based on McMaster’s initiative to slowly phase in research. During this time the Centre will only be open in person to staff members, but all users can access the CCEM by sending in their samples to the staff. Every effort will be made to connect remotely with users during the microscope sessions. To plan a project or submit a sample to the CCEM, it is requested that you contact CCEM management or respective staff member.  If you have any further questions regarding our limited re-opening please contact the Acting Manager.

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2020 CCEM Art Gallery Competition

The CCEM will be holding its third annual image competition to fill our art gallery! The images can be black and white or have false colouring added; it is up to your artistic discretion.

Requirements:
-Please send images in tiff format to ccemreg@mcmaster.ca with the following file extension “firstname_lastname_art”. In the email indicate:

  1. your affiliation
  2. the name of your art piece
  3. the material imaged
  4. the name of the microscope operator (if applicable)

-Only one image can be submitted per user! Image may not have been submitted in previous competition.
-The images must be acquired at the CCEM
-The submitted images should be 11.75 inches x 11.75 inches (width x height)

Deadline: May 11, 2020

Selection Process:
A vote will then be had by all CCEM users to select the top six images that will be featured in the art gallery.

Finalists will be featured at the CCEM User meeting and in the CCEM art gallery for the 2020 season!

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CCEM closed until Wednesday March 25, 2020

Due to the dynamic evolution of the COVID-19 virus, we have made the hard decision to close the CCEM until March 25, 2020. After this one week closure we will reassess the situation and send an update to all users. Should you have any questions or concerns please contact Andreas Korinek, the facility manager via email. Team members will still be accessible via email to answer your questions regarding research.

The CCEM team wishes you all the best for the coming weeks and we are looking forward to providing you with in person training and on-site access to the instruments as soon as the situation allows.

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User access restricted to CCEM until further notice

Due to the current difficult circumstances, CCEM will not allow students and visitors to the facility effective immediately until further notice. CCEM will remain open and staff will be on site to analyze samples. If you have samples to be submitted, please use NanoLIMS to create a job. You can either send samples by courier or mail or use the drop-box at the front entrance of the facility. Andreas Korinek, the facility manager can be contacted via email for inquiries and questions. CCEM will waive staff costs for samples submitted until further notice.

The CCEM team wishes you all the best for the coming weeks and we are looking forward to providing you with in person training and on-site access to the instruments as soon as the situation allows.